Epitaxial Layers on Oxidation-Sensitive Substrates and Method of Producing Same

ABSTRACT

This invention disclosure describes methods for the fabrication of metal oxide films on surfaces by topotactic anion exchange, and laminate structures enabled by the method. A precursor metal-nonmetal film is deposited on the surface, and is subsequently oxidized via topotactic anion exchange to yield a topotactic metal-oxide product film. The structures include a metal-oxide layer(s) and/or a metal-nonmetal layer(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/997,014, filed on Nov. 24, 2004, and now pending, the entiredisclosure of which is incorporated herein by reference. Priority tothis application is claimed.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

TECHNICAL FIELD

This invention relates generally to barrier layers used inelectromagnetic and other device structures and methods of forming such,and more specifically to the provision of epitaxial, textured,crystalline, or polycrystalline oxide layers on surfaces for which itmay be difficult to fabricate such layers.

BACKGROUND OF THE INVENTION

The manufacture of a great variety of electromagnetic and opticaldevices is based upon thin film technology. A succession of layershaving various functionality are deposited on a planar substratesurface, one on top of another. Each is patterned in some manner,resulting in a complex three-dimensional device such as an integratedcircuit. It is this technology, called metal-oxide-semiconductor (MOS)in the case of its use on doped silicon, that has enabled the computerrevolution of the late 20th century, which continues today. Anotherexample of thin film technology is the current effort to developsuperconducting power transmission cables, using the coated-conductortechnique. Thin films are also useful on substrates that are non-planarsurfaces, such as their use as templates for oxide environmentalbarriers on three-dimensional objects.

There exists a wide array of crystalline oxide materials that havespecial or exceptional properties, and that are highly desirable asfunctional layers in thin-film devices. These properties includecolossal magnetoresistance, ferroelectricity, superconductivity, verylow thermal conductivity, and high dielectric constant, and many others.

Two major difficulties exist with the integration of these materials.First, most of these functional materials are oxides. And, as it turnsout, the materials commonly or most conveniently used as a flatsubstrate surface are often sensitive to reaction with oxygen, such assilicon, gallium arsenide, nickel, or copper. The functional materialcomponents may be reactive with the substrate in other manners as well.Thus, it is extremely difficult to deposit highly-desirable functionaloxides directly on these substrate surfaces. Barrier layers, used toblock oxygen or other ionic interaction with the substrate, are used inmany instances with success, although they are often complicated.Therefore more economical solutions are desirable.

The second difficulty is the need for crystalline templating. A largeproportion of these functional oxides are most preferably deposited as asingle-crystal-like film. That is, the crystalline material of thehaving a particular crystalline orientation and texture. The substratesused for deposition are frequently monocrystalline (e.g., silicon) ormonocrystal-like (epitaxial, fiber-textured platinum, biaxially texturednickel), although polycrystalline surfaces are also frequently employed.If it was possible to deposit functional oxides directly on substrates,this substrate crystallinity could be used to encourage their growth ina single-crystal-like form, i.e., epitaxial growth. Epitaxial growth offunctional oxides is routinely conducted, but usually only on oxidesurfaces. Due to oxidation sensitivity of non-oxide substrates,epitaxial growth is prohibited in all cases but those with the moststringent growth conditions, and then for only a few specific systems.

These two difficulties can be addressed individually with reasonablesuccess, but a method that addresses both of these difficultiesconcurrently is elusive. Barrier layers can be used, but are typicallynot single-crystal-like, being either amorphous (glassy) orpolycrystalline. The growth of single-crystalline oxides directly onthese substrates is exceedingly difficult.

This invention addresses this problem directly. It discloses a method,compositions, and the resulting devices for the fabrication ofsingle-crystal-like oxide layers on oxidation-sensitive substrates,whether fabricated by the disclosed method or another. Moreover, themethod is straightforward, and can be performed with far less difficultythan that encountered in direct growth of oxides on such substrates.Devices, optimally fabricated by the method, and using compositionsoptimal for an application, provide a templating surface that can betreated in essentially the same manner as a crystalline oxide surface.Templating is transmitted from the substrate to the film surface, andoxygen interaction with the substrate is blocked. Thus, the growth offunctional oxides is made straightforward, which then can be conductedin essentially the same manner as is routinely used for their depositionon monocrystalline oxide substrates.

As an added benefit, an oxide film that the use of this method providesmay also be a functional oxide in and of itself. In cases such as this,the complexity of the fabrication process is further reduced.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a method of fabricating ametal oxide film upon a surface is provided. A substrate having asurface is provided. A crystalline metal-nonmetal precursor film isdeposited on the surface via a physical vapor deposition method. Theprecursor film comprises metal and nonmetal constituents. The metalconstituents of the precursor film are selected from the groupconsisting of the lanthanide elements (Ln): Sc, Y, La, Ce, Pr, Nd, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and combinations, alloys, anddoped substituents thereof. The nonmetal constituents of the precursorfilm are selected from the group consisting of: H, C, N, F, P, S, Cl,Se, Br, Te, and combinations thereof. The precursor film is oxidized toyield a metal-oxide product film by heating to a temperature between 0and 1000° C. in a controlled environment and introducing an oxidizingagent into the environment. The metal constituents of the product filmare those of the precursor film. A majority of the nonmetal constituentsof the precursor film are exchanged for oxygen to yield the productfilm. The product film is topotactic with the precursor film.

In another embodiment of the present invention, a functional laminatestructure is provided. The structure includes a substrate having asurface, a crystalline metal-nonmetal layer, and a crystallinemetal-oxide layer. The metal-nonmetal is selected from the groupconsisting of AX_(0.3), AX, AX₂, AX₃, and compounds consisting ofcombinations of said materials. The metal-oxide layer is topotactic withsaid metal-nonmetal layer. The metal constituents of the metal-oxidelayer are those of the metal-nonmetal layer. The metal-nonmetal layer islocated between the surface and the metal-oxide layer.

In yet another embodiment of the present invention, a functionallaminate structure is provided. The structure includes a substratehaving a surface, a crystalline metal-nonmetal layer, and a crystallinemetal-oxide layer. The metal-nonmetal is selected from the groupconsisting of: ABX₃, A_(n+1)B_(n)X_(3n+1), doped substituents thereof,anion-deficient versions thereof, and anion-excess versions thereof. Themetal-oxide layer is topotactic with said metal-nonmetal layer. Themetal constituents of the metal-oxide layer are those of themetal-nonmetal layer. The metal-oxide is selected from the groupconsisting of ABO₃, A_(n+1)B_(n)O_(3n)+₁, doped substituents thereof,anion-deficient versions thereof, and anion-excess versions thereof. Themetal-nonmetal layer is located between the surface and the metal-oxidelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-resolution transmission electron microscope image of aTbO_(z) crystal down the [211]_(F) axis, demonstrating the coexistenceand topotactic compatibility of the different-m members of thehomologous series Ln_(m)O_(2n-2m).

FIG. 2A is a high-resolution transmission electron microscope image of aPrO_(z) crystal, demonstrating the coexistence and topotacticcompatibility of the different-m members of the homologous seriesLn_(m)O_(2n-2m).

FIG. 2B is a digital diffraction pattern of a PrO_(z) crystal of thesingle phase area labeled A in FIG. 2A, showing a common <110>_(F)sublattice with the same orientation.

FIG. 2C is a digital diffraction pattern of a PrO_(z) crystal of thesingle phase area labeled B in FIG. 2B, showing a common <110>_(F)sublattice with the same orientation.

FIG. 2D is a digital diffraction pattern of a PrO_(z) crystal of thesingle phase area labeled C in FIG. 2C, showing a common <110>_(F)sublattice with the same orientation.

FIG. 2E is a digital diffraction pattern of a PrO_(z) crystal taken froma small region of the phase area labeled B in FIG. 2C.

FIG. 3 is a stability diagram for polymorphic forms of Ln₂O₃, showingthat the C-type form is observed at room temperature for all Ln₂O₃.

FIG. 4 is x-ray diffraction data showing topotactic transformation of(100) TiN/(100) Si to (110) TiO₂/(100) Si with low texture quality. Theθ-2θ pattern demonstrates the presence of only the describedorientations and phases in the same film before and aftertransformation, and the pole figures demonstrate the textural quality ofthe TiO₂ film, which is poor due to the mosaic and porosity caused bythe change in lattice dimension upon anion exchange, but is stilltopotactic.

FIG. 5 is x-ray diffraction data showing topotactic transformation of(100) (Zr,Y)N/(100)_(biax)Ni to (100) (Zr,Y)O₂/(100)_(biax)Ni with lowtexture quality. The scans show only the presence of 001-orientedmaterial. Transformation-induced mosaic texture of the (Zr,Y)O₂ film is4.8° out-of-plane, and 11° in-plane.

FIG. 6A is a cross-sectional transmission electron microscope image ofthe full film thickness, showing the interface between (Zr,Y)N and(Zr,Y)O₂.

FIG. 6B is a high-resolution transmission electron microscope image ofthe interface between (Zr,Y)N and (Zr,Y)O₂, and diffraction patterns ofthe same area, demonstrating topotaxy.

FIG. 6C is an electron diffraction pattern of the (Zr,Y)O₂,demonstrating topotaxy with the (Zr,Y)N.

FIG. 6D is an electron diffraction pattern of the (Zr,Y)N in the samearea, demonstrating topotaxy.

FIG. 7 are unit cells of rocksalt-type, C-type (bixbyite), andfluorite-type lanthanide oxides. The common structural element, an FCCcation sublattice, is clear in the images. Dark circles are Ln, whiteoxygen. The structures differ only in the distribution of the oxygenions.

FIG. 8 shows the crystal structures of Ln₂O₃ and LnO₂. C-type andfluorite-type LnO_(z) are amenable to the topotactic anion exchangemethod, but A-type and B-type Ln₂O₃ are not.

FIG. 9 demonstrates the multitude of monoclinic and other complex unitcells for Ln_(m)O_(2n-2m) are all based on a nearly undistorted cationsublattice, as demonstrated here by the superposition of severalmonoclinic unit cells over a mesh of FCC cations, viewed here along<110>.

FIG. 10 is a plot of the lattice parameter of several Ln_(m)O_(2n-2m),showing that transformation between the phases is smooth, with no sharpchanges in lattice dimension that would induce dislocations, pores, orcracking, demonstrated here for Pr_(m)O_(2m-m) (a subset of theLn_(m)O_(2n-2m) homologous series).

FIG. 11 is a plot of the lattice parameter of the pseudocubic subcell ofnumerous Ln₂M₂O₇. The fit to several typical substrates is also shown.

FIG. 12 is the structure of EuWO₄, exhibiting the scheelite structure.The cation lattice is FCC, alternating Eu and W along <110>. Tetrahedraare W, with oxygen at apices, and circles are Eu.

FIG. 13 is the observed LnTrO₄ crystal structures. Those havingfergusonite, wolframite, or scheelite structures have FCC or slightlydistorted FCC cation sublattices, and are amenable to topotactic anionexchange.

FIG. 14 is a plot of the lattice parameter of the pseudocubic subcell ofLnO_(z). The fit to several typical substrates is also shown.

FIG. 15 is a plot of the lattice parameter of the pseudocubic subcell ofnumerous LnX_(y). The fit to several typical substrates along astructurally-compatible axis is also shown.

FIG. 16 is a plot of the change in lattice parameter of the pseudocubicsubcell of numerous LnO_(z) upon topotactic conversion from LnX_(y).

FIG. 17 is a plot of the relative change in lattice parameter of thepseudocubic subcell of numerous LnO_(z) upon topotactic conversion fromLnX_(y).

FIG. 18 is a plot of the lattice parameter of the pseudocubic subcell ofCeO₂-Ln₂O₃ alloys, demonstrating the continuously variable range oflattice parameters achievable using the approach. Most of the two-phaseregions are topotactically compatible.

FIG. 19 is a plot of the lattice parameter of the pseudocubic subcell ofZrO₂-Ln₂O₃ alloys, demonstrating the continuously variable range oflattice parameters achievable using the approach. Most of the two-phaseregions are topotactically compatible.

FIG. 20 is a plot of the change in electrical conductance withtemperature for reduced CeO₂. Electronic defects are relaxed afteranneal. This type of measurement is ideal for determining thetemperature appropriate for topotactic anion exchange for a givensystem.

FIG. 21 is an abbreviated plot of formation energies for some LnO_(z),in comparison to common oxides. They are similar in this respect tocalcium or aluminum.

FIG. 22 are exemplary descriptions of device structures. A lanthanidesalt precursor can be deposited on a surface that is epitaxial,biaxially textured, fiber textured, amorphous, or polycrystalline. Theprecursor can be partially or fully converted, or an oxide-salt alloycan be deposited and either converted or used as deposited. The cationicconstituents can be lanthanide(s), or a mixture of lanthanide(s) withother metal(s). Abbreviations in the figure are as follows:amorph—amorphous, biax—biaxially textured, epi—epitaxial, fiber—fibertextured, Ln—lanthanide or lanthanides, M—metal or metals,polyx—polycrystalline, single xtal—monocrystalline, topo—topotactic,X—nonmetal constituent or anion or anions, xtal—crystalline.

DETAILED DESCRIPTION

Headings in the detailed description are meant solely as a guide for thereader in finding sections of interest, and are not meant tocharacterize or to limit the invention in any way.

The focus of this disclosure is lanthanide oxide andlanthanide-containing oxide thin films, preferably as formed by themethod of topotactic anion exchange. Device fabrication by this methodis not known in the art and is demonstrated below. The method enablesfabrication of many new devices. This method and devices fabricatedthereby find use as epitaxial oxide templates, barrier layers, andvarious functional oxide layers.

Lanthanide salts (lanthanide metal-nonmetal compounds) andlanthanide-containing salts have a specific type of crystal structurefor which the nonmetal components can be exchanged for oxygen with nodisruption of crystalline texture—the cation lattice is undisturbed bythe anion exchange, such that the “product crystals have inherited theirorientations from the reagent crystals.” That is, the productlanthanide-oxide is topotactic with the precursor lanthanide-nonmetal;it has an orientation reproducibly related to that of the precursor,regardless of whether the precursor is completely or partially convertedto the oxide. The phenomenon is termed topotactic anion exchange.

Topotactic anion exchange functions in basically the same manner forlanthanide-containing thin films as for single crystals, but withadditional advantages over single crystals. Lanthanide-containing saltsand oxides exhibiting a face-centered cubic (FCC) cation sublattice arethe most suitable materials for a topotactic anion exchange process forthe fabrication of oxide thin films. Although two isolated,serendipitous observations of topotactic anion exchange in transitionmetal oxide thin film systems are available in the art, such systems arenot suitable for the fabrication of devices having a high degree ofcrystalline and microstructural perfection, and none containinglanthanides have been reported or proposed. Indeed, others in the artconsider the two mentioned examples as unique systems. The reducedphysical dimension of thin films and the physical constraint of topotaxyand enhance the stability of topotactically converted materials, suchthat the range of compositions for which the method is applicable isgreater than that for bulk and single crystalline systems, and aninfinite variety of alloyed compositions is possible, enabling tailoringof lattice dimension and other properties for any given application.

A. Overview

This invention concerns primarily oriented and epitaxial oxide films onsubstrates, particularly on substrates that are sensitive to reactionwith elemental constituents of the film. It consists of a method andnumerous devices which are enabled by the method. The most significantapplication for this method is the use of these oxide films as epitaxialoxide template layers, for the subsequent growth of other oxide layerson the oxide-templated surface of oxidation-sensitive substrates, thesensitivity being such that it is impossible or very difficult andexpensive to deposit epitaxial oxides directly on the surface of thesubstrate.

A critical aspect of the method is that the precursor to the barrierlayer is chosen to be effectively stable in contact with the substratesurface at the time of its deposition. It is subsequent to thedeposition (or at least after the initial stage of its deposition) thatthe layer is oxidized to yield a layer that is effectively stable withoriented films deposited thereon, which can additionally act as an oxidetemplate to control the orientation of films deposited thereon.

The template films consist of lanthanide oxides, or crystalline phaseswith a major component being a lanthanide oxide. These template oxidefilms are fabricated by first depositing heteroepitaxial precursor filmscomprising an R—Zr family metal (scandium, yttrium, zirconium, hafnium,and the proper lanthanide elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, and Lu), or some combination thereof, with or withoutother elements as major constituents). As a matter of notation, the R—Zrfamily of metals is hereafter referred to as a group as “lanthanides” orby “Ln,” or nonexclusively as “cations” or “metal” in the context ofcompound constituents, and “nonmetal constituents” or “anions” (H, C,(OH), O, N, F, P, S, Cl, Se, and Br; and Te, or some combinationthereof), are hereafter referred to as a group as the “anions,” “X,” orthe “nonmetal” component, to yield a composition LnX_(y) (y indicatesthe stoichiometry of X relative to Ln). The substrate surface may besingle crystalline (e.g., a silicon crystal or an epitaxial film on acrystal), biaxially textured (e.g., nickel), fiber textured (e.g.,platinum), polycrystalline, or amorphous. The deposited LnX_(y) phasehas a rigid cation sublattice, preferably face-centered cubic (FCC),which is rigid at temperatures below ˜1200° C., enabling high mobilitiesof anionic species concurrent with retention of crystalline integrity,texture, and orientation. The LnX_(y) film is subsequently exposed at anelevated temperature to an oxidizing agent, such that the anionicspecies are replaced through evaporation and in-diffusion of oxygen, toyield an epitaxial LnO_(z) film (z indicates the stoichiometry of oxygenrelative to Ln). The exchange is diffusionally topotactic, that is, thecrystalline lattice is maintained, specifically the cation sublattice,such that the resulting epitaxial LnO_(z) film has an orientation andtexture related to that of the precursor LnX_(y) film.

Oxide films may then be deposited under normal conditions directly onthe LnO_(z) template film surface, as they would on any other oxidesubstrate. Any subsequent oxidation of the substrate, by oxygendiffusing through the LnO_(z) during subsequent depositions on itssurface may result in a slight, even, and insignificant oxidation of theunderlying substrate, but because the orientation and texture of thefilm is established prior to this, orientation of the oxide template andfilm are unaffected, having already established templating by theLnO_(z) surface presented by the template layer. Numerous devicesemploying LnO_(z) as an epitaxial oxide template layer or as afunctional material are possible.

While the product LnO_(z) film will typically be arocksalt-structure-based or fluorite-structure-based phase (fluorite,bixbyite, pyrochlore, scheelite, etc.), it is important to note that theprecursor film may also contain oxygen as a component, typically insolution with other anions, to the degree that films can be depositedwithout undesirable reaction with the underlying substrate and itssurface. This consideration can reduce the oxygen partial pressurerequired for deposition of a templated barrier layer significantly. Itis also important to note that an FCC cation sublattice is preferred butnot necessary for the method. Other oxide phases, such as those withface-centered or close-packed slab layers, or with BCC cationsublattice, having lanthanide(s) as a major component, may also bepossible by the method.

Heteroepitaxial growth, the growth of one phase on a surface of another,is widely used to produce films of many compositions. Heteroepitaxialgrowth of non-metal films is typically performed using the same orsimilar anion species in both substrate and film, for example YBa₂Cu₃O₇films grown on SrTiO₃. Difficulty is encountered in attempts to depositfilms on substrates having differing components. Both kinetic andthermodynamic limitations exist. Elevated temperatures are needed toprovide sufficient thermal excitation of metallic and cationic speciesto achieve high-quality epitaxial films, approximately in the range of300-1200° C. At elevated temperatures, anionic species are highlymobile, which can lead to intermixing and other issues. Thethermodynamic conditions necessary for the formation of the desiredphase in the film are often such that the elemental constituents of thefilm will react with the substrate as well. That is, the substrate issensitive.

It is not possible to overcome directly thermodynamic limitations, butit is possible to side-step the limitation by avoiding directdeposition, frequently pursued in the form of barrier layers. Manycomplicated layering schemes have been devised for various combinationsof substrate and film composition, for which each layer is stable incontact with the layer directly underlying it. Such an approach can becumbersome. Multiple depositions incur added expense for processes thatwould potentially incorporate such schemes, and the use of manydifferent and possibly incompatible schemes increases processcomplexity. A simpler method is desirable.

The method of this disclosure employs only a single deposition of abarrier layer that is chemically stable in contact with the substrate atthe time of barrier-layer deposition, and after a brief oxidationanneal, is chemically stable in contact with any epitaxial oxide filmdeposited thereon at the time of its deposition. Preferred embodimentdevices consist of a film, containing lanthanide species as (a) majorcomponent(s), present on a crystalline substrate that conferscrystalline orientation and surface mesh characteristics to its surface,and which can act as an intermediate layer to prevent reaction of thesubstrate with the environment or with subsequent film layers. In apreferred embodiment, the film has been partially converted from theprecursor such that the region in contact with the substrate remainsstable in contact with the substrate, and the free surface is an oxide,suitable for the deposition of other oxide films. The device is usedeither as a functional layer in a device, or as a surface used for thedeposition of oxide films having other compositions which are functionallayers in a device.

The invention relies on the well-known behavior of lanthanide oxides tohave a rigid cation structure at all temperatures below their meltingpoint, around 2000° C., which is exceptional among oxides. The basallattice parameter of lanthanide ionic salts (lanthanide metal-nonmetalcompounds) and oxides are similar. The crystalline lattice is notcontrolled by oxygen packing, but by a combination of anion and cationpacking. Several lanthanides also have multiple oxidation states, andtherefore several possible oxide compositions. The majority of the manydifferent oxide structures for single lanthanides are based upon thefluorite and rocksalt crystal structures, but with differences in oxygencontent, oxygen vacancy ordering, and slight (i.e., insignificant withrespect to epitaxy) shifts in the atomic positions of the cations. Thesephases also have appreciable oxygen diffusivities. But for all of theseoxides in the solid state, the cations remain in a rigid face-centeredcubic (FCC) structure, with positions and coordination essentiallyconstant. The majority of multi-component lanthanide oxides are alsoFCC-based and behave in the same manner. Some multi-component lanthanideoxides have a body-centered cubic (BCC) structure, for which the cationlattice is also rigid. The cation sublattice in the differentlystructured lanthanide oxides of A-type, and B-type structures is alsorigid, although not FCC.

Lanthanide oxides are not the only materials to exhibit this behavior.Many LnX_(y) have structures having an FCC cation sublattice, withrocksalt-based or fluorite-based crystal structures, and frequentlyseveral. These exhibit the same type of behavior described above foroxides. The structures consist of a rigid FCC cation sublattice, withanions distributed in various manners in the FCC cation lattice, andanion diffusion rates are frequently appreciable. Alloy mixtures of manyof these compounds are known.

The commonality between these two types of lanthanidecompounds—fluorite/rocksalt-based oxides, and rocksalt/fluorite-basedsalts—is that the cation lattice is FCC and is rigid. It is the same inboth cases. (Minor position distortions occur, but the FCC cationcoordination remains.) Solid solutions between these materials are alsowell known, such that substitution for one or several differentlanthanides in a single phase, or of several anions in a single phase,or even a solution consisting of several different lanthanides andseveral different anions, are possible.

It is possible to exchange the anionic component of a LnX_(y) crystalfor oxygen. The pure oxide is frequently the most chemically stable ofthe compositions, so exchange of other anions for oxygen isenergetically preferred and straightforward. Also due to the high aniondiffusivities, this exchange does not disrupt the quality of thecrystal. No cation diffusion is required. This sort of chemical exchangeresults in topotaxy, more specifically it is a topotactic anion exchangereaction. Individual crystals of the product oxide have an orientationreproducibly related to that of the precursor. This type of reaction canoccur under more limited conditions for lanthanide phases having a BCCcation sublattice, and under other limited conditions for A-type,B-type, and hexagonal phases as well.

In many cases, it is much easier to deposit an LnX_(y) film of thedesired orientation on a particular substrate than it is to deposit anLnO_(z) film. This is in large part a result of the oxidationsensitivity of the substrate, precluding the deposition of an oxidedirectly on its surface, as such deposition typically requires thepresence of an oxygen-bearing gas and elevated temperatures.

It should be pointed out that, depending upon the diffusionalcharacteristics of the LnO_(z) film, the underlying substrate may beoxidized at a later time, due to an in-diffusion of oxygen through thetemplate layer. This may in some cases cause difficulties, as functionalstructures that are entirely epitaxial may in some cases be verydesirable, but in many cases it will not. Interface oxidation below thelayer may be irrelevant. Additionally, it is possible to use severallayers or a graded structure for the layer hereby produced. Suchstructures can be designed to avoid post-conversion substrate oxidation,if such is desirable.

An LnO_(z) surface presents an oxide template that is amenable to the(locally) epitaxial deposition of a large number of technologicallysignificant oxide materials. For heteroepitaxial film deposition onthese surfaces, the LnO_(z) epitaxial oxide template films present acation surface corresponding to the FCC cation arrangement, on whichoxygens can stably adopt an arrangement corresponding to theirarrangement in either fluorite-based or rocksalt-based phases. For abasal surface, that is either a square surface net with a period equalto 1/SQRT(2) of the FCC sublattice dimension, or a close-packed surfacenet, with a period equal to the FCC sublattice dimension, depending onconditions such as temperature and oxygen partial pressure. For example,numerous perovskite-based materials are technologically important. The(100) surface of an oxide perovskite presents a surface net that matchesboth the cation surface net, and the oxygen surface net offluorite-based lanthanide oxides, Ln₂O₃, so epitaxy is readily achievedfor such films grown on LnO_(z). Additionally, many of LnO_(z) phasesare tolerant to some proportion of oxygen vacancies, thus the interfacestructure must not necessarily match exactly, and a single atomic layerof intermediate oxygen coordination can exist between the LnO_(z)template and the desired epitaxial oxide film.

Potential uses of such layers primarily include enabling the economicalgrowth of epitaxial or oriented layers that provide specificfunctionality, although such layers may themselves be functional. A fewexamples of structures enabled by this method and device include (1) thegrowth of silicon on an insulating layer on silicon, for dielectricisolation of silicon-on-insulator devices, (2) ferroelectric oxides onsilicon for use in capacitor elements or field-effect devices, (3)growth of superconductors on textured nickel for coated conductor powertransmission cables, and (4) the deposition of ahigh-dielectric-constant material on silicon for use in field-effect andcapacitive devices.

B. Thin Film Technology

The manufacture of a great variety of electromagnetic and opticaldevices is based upon thin film technology. A succession of layershaving various functionality are deposited on a planar substratesurface, one on top of another. Each is patterned in some manner,resulting in a complex three-dimensional device such as an integratedcircuit. It is this technology, called metal-oxide-semiconductor (MOS)in the case of its use on doped silicon, that has enabled the computerrevolution of the late 20th century, which continues today. Anotherexample of thin film technology is the current effort to developsuperconducting power transmission cables, using metal ribbons assubstrates. Thin films are also useful on non-planar surfaces, such astheir use as environmental barrier devices on three-dimensional objects.

There exists a wide array of crystalline materials that have special orexceptional properties, and that are highly desirable as functionallayers in thin-film devices. These materials include colossalmagnetoresistive materials, ferroelectrics, superconductors, and high-kdielectrics, piezoelectrics, insulators, and many others.

Two major difficulties exist with the integration of these materials.First, most of these functional materials are oxides, but the materialscommonly or most conveniently used as a substrate surface for depositionare often very sensitive to reaction with oxygen, such as silicon,nickel, gallium arsenide, or copper. Second, crystalline templating isfrequently needed to yield films having a specific orientation ortexture. Oxidation of a substrate surface during the initial stages offilm deposition can destroy this templating.

C. Barrier Layers

Barrier layers are frequently used in thin film fabrication processes toprevent chemical interaction or interdiffusion of the chemical speciesin separate layers. Many various and often complicated layering schemeshave been devised for various combinations of substrate and filmcomposition, for which each layer is stable in contact with the layerdirectly underlying it. Such an approach can be cumbersome. Multipledepositions incur added expense for processes that would potentiallyincorporate such schemes, and the use of many different and possiblyincompatible schemes increases process complexity.

One example barrier layer architecture is that of an amorphous(Hf,Zr)N_(x) layer on silicon, and an amorphous (Hf,Zr)(O,N)_(x), or(Hf,Zr,Si)(O,N)_(x) layer on silicon, used as the gate oxide in afield-effect transistor device, with a conductive metal thereon, used asa method for the fabrication of a field-effect transistor. A simplermethod is desirable.

Barrier layer architectures introduce further problems which must alsobe addressed in addition to the stability issue. For example, lack ofadhesion is commonly encountered. This results in further increasingcomplexity in design of the barrier layer architecture. For example, adesign in the art addresses this problem through the addition of extralayers designed to promote similar chemical bonding and adhesion betweenlayers. The structure consists of a metal top electrode on a functionaloxide perovskite layer on a Ru or Ir-containing perovskite layer on aRuO₂ or IrO₂ layer on a metal Ir or Ru layer on a substrate. Such astructure, six layers, increases costs significantly. A simple singlelayer is preferred for a barrier layer.

D. Epitaxy

There exists a large variety of oxide materials with useful properties,which have many different uses. For example, for the semiconductorindustry, integrating various oxide insulators, dielectrics,ferroelectrics, superconductors, magnetoelectrics, and other materialswith existing semiconductor fabrication processes is an extremelyattractive and desperately needed goal. Some superconducting powertransmission cable technology is based upon the use of metal ribbons,intended to be used as a substrate for the deposition of epitaxial(c-axis) YBa₂Cu₃O₇ superconductor using appropriate buffer layers. Someturbines require the deposition of barrier layers to prevent excessiveoxidation in hot spots.

In all of these cases, in addition to substrate-oxidation issues,deposition of functional epitaxial oxides are desirable. Deposition oftextured or epitaxial films can increase the range of oxide films thatcan be integrated in a useful manner. For example, many oxides arehighly anisotropic, and therefore must be grown in a specific epitaxialorientation to be useful. Many substrates are single crystalline, makingthem suitable for epitaxial film growth. Many substrates are biaxiallytextured, fiber textured, or polycrystalline, and epitaxial film growthon such surfaces, in which grains are locally epitaxial to individualgrains of the surface, results in a film having similar texture. Inaddition to textured substrates, there are numerous cases where an oxidebarrier is desirable on polycrystalline or amorphous substrates. Casesexist for which both textured and polycrystalline oxide template layersare desirable.

Because these cases involve local epitaxy of the deposited film, theterm “epitaxial” will be frequently be used to describe both situationsin the context of film growth. Even growth on a polycrystallinesubstrate can be locally epitaxial. A topotactic anion exchange reactionenables texture retention regardless of precursor texture.

There have been recent demonstrations of epitaxial film growth directlyon oxidation-sensitive substrates, but this is possible only for a smallsubset of useful oxides, or under stringent conditions, and theprocesses expensive, making an alternative solution preferable, and onlyin specific orientations. For example, although some LnO_(z) can bedeposited epitaxially on silicon, only (110) orientation is observed,and special conditions must be employed to obtain single-domain growthof that single orientation.

1. Epitaxial Barrier Layers

Thus, it is clear that a desperate need exists for an oxide templatebarrier layer for many different applications and chemical systems,especially in epitaxial systems. Isolated specific examples of epitaxialbarrier layers exist in the art, but these typically increase processcomplexity significantly.

For example, a previously described epitaxial barrier layer stackstrategy to provide a specific type of epitaxial oxide surface onsilicon consists of either a wurtzite-type oxide on a R—Zr oxide on alanthanide manganite layer on an aluminum oxide layer on silicon, or ofa wurtzite-type oxide on a R—Zr oxide on a lanthanide manganite layer onan aluminum oxide layer on a tantalum nitride, titanium nitride, orniobium nitride layer on silicon.

The above-described layer structure relies on a manganite layer, whichis likely to be magnetic, and could introduce problems in anelectromagnetic device. Additionally, the layer structure is one that islikely to be unstable over time, as tantalum and niobium nitride layersare known to form extensive porosity upon exposure to oxygen andelevated temperatures. The deposition of any oxide layers on thisstructure would result in such oxidation, and would be destructive tothe physical integrity of the stack. Furthermore, the wurtzite surfacelayer of such a structure is hexagonal, and on a typical (100) siliconsurface will consist of many domains, the walls between which are fastdiffusion paths for impurities. In contrast, this disclosure describesan oxide barrier layer that can consist of only a single layer to bedeposited, or multiple layers if appropriate or desired.

Another epitaxial oxide template and barrier device consists of astructure comprised of a metal oxide layer comprised of an ABO₃perovskite material deposited directly on a single crystalline siliconsurface, with further layers of functional oxides. This type ofdeposition requires an exceptional degree of finesse, because thethermodynamic conditions (temperature, partial pressure of constituentgases) for deposition are very close to those resulting in oxidation ofthe substrate, silicon in this case. In fact, silicon will oxidizebefore the elemental constituents of may perovskite compositions,effectively preventing successful use of this approach.

E. Topotaxy

An alternative to the direct deposition of oxides on sensitivesubstrates for the fabrication of epitaxial template and barrier layersis highly desirable. Topotaxy offers this alternative method by enablingthe deposition of a precursor of desired crystalline texture that isthermodynamically or kinetically stable in contact with the substrate atthe time of deposition, and which is stable in contact with materialslater deposited thereon at the time of their deposition. In other words,a precursor layer is transformed to an oxide template barrier layersubsequent to deposition on the oxidation-sensitive substrate.

The method of this disclosure relies on the phenomenon of topotaxy,specifically the topotactic exchange of anions in inorganic,lanthanide-containing systems, as detailed below.

1. Definition

Topotaxy indicates that “the product crystals have inherited theirorientations from the reagent crystals,” characterized by “a definiteand reproducible crystallographic orientation relationship between thetwo,” and which also involves a change in composition (presumablythrough out-diffusion or in-diffusion of one or more constituentspecies). Reactions resulting in topotaxy are referred to as topotacticreactions.

Topotactic solid-state reactions occur in systems for which one of thecomponents has a very high mobility in the solid, while the other is arigid or constrained, over some specific range of temperatures. If sucha material is exposed to a source of some other component elements, forwhich a compound with the non-diffusing species is more energeticallypreferred, and for which the source component also has a high mobilityin the material, then a topotactic reaction can occur. A topotacticreaction results in a product phase that is distinct from the parentphase.

2. Types

Topotaxy has been known for almost a century, and topotaxy in metallic(Widmanstätten reactions) and organic systems is widely known, but willnot be addressed here. For inorganic systems, four types of topotacticreactions exist.

a. Persistent Slab—Intercalation/Deintercalation

Persistent slab topotactic synthesis and exchange methods are differentfrom the process in this disclosure. These involve layered or sheet-likecompounds (e.g., graphite, clays, or Ruddlesden-Popper phases), and thecation exchange in these systems is more properly referred to astopochemical, or an intercalation/deintercalation reaction. A rigid slabof the parent phase, for example a perovskite double layer of a layeredperovskite phase, remains unchanged, while the components between theslabs are exchanged.

An example of persistent slab topotaxy is the incorporation of CuCl orCuBr into the n=2 Dion-Jacobson phase (layered perovskite) RbLaNb₂O₇. Aparent phase is annealed at 325° C. for seven days in the presence ofCuCl₂ or CuBr₂. The very large size of the Rb atoms, which lie betweenthe double-perovskite sheets enables the exchange. Such an exchangeresults in the disintegration of a single crystal or bulk parent phaseupon reaction, as the lattice expands by 6.4% in the c direction uponconversion.

Some topotactic work has been carried out in a manner referred to assoft chemistry or exfoliation. In these processes, a parent phase havinga structure composed of rigid and chemically stable slab layersalternating with alkali or alkaline earth layer is used. The alkali oralkaline earth is removed through solution chemistry processes,resulting in an exfoliation process. The slabs, floating in a solution,can be caught and manipulated in various manners, such as layer-by-layerassembly techniques. This has been used as a method to produce texturedTiO₂ and BaTiO₃ films from a Ti_(1.73)O₄H_(1.08) precursor.

b. Cation Exchange (Persistent Anion Sublattice)

Persistent anion sublattice topotactic reactions are the most widelystudied in oxides. Systems are typically transition metal oxides, forwhich the oxygen sublattice is close-packed, and cations are mobileenough to allow for exchange. Most examples involve the formation ofspinels or the breakdown of some hydrated silicates. Similar reactionshave been reported in organic systems. In both parent and product, theoxygen lattice is close-packed.

The reaction of SnO vapor with MgO yields a topotactic layer of Mg₂SnO₄spinel on MgO. Very high temperatures, 1200-1300° C., are required toachieve sufficient mobilities for this reaction to occur. A similartopotactic reaction occurs for solid-solid reaction of the TiO₂—MgOsystem to yield Mg₂TiO₃ and Mg₂TiO₄ spinel. The NiO—Al₂O₃ systemundergoes the same type of transformation, with the {111}-type Ni₂AlO₄planes parallel to the (0001) Al₂O₃ (hexagonal) planes. Solid-solidreaction of MgO and Nb₂O₅ or Ta₂O₅ at 800° C. formscorundum-type-structured Mg₄Nb₂O₉ or Mg₄Ta₂O₉. β-Ni(OH)₂ transforms toβ-NiOOH upon heating to evolve hydrogen.

c. Anion Exchange (Persistent Cation Sublattice)

Anion-exchange topotaxy is much less widely known, and is the focus ofthis disclosure. In this case, the cation lattice is rigid andpersistent, and anions have high mobility. Numerous lanthanide inorganicphases fit these two criteria.

Taking for example the oxides, LnO_(z), the anion substructure “melts”far below the physical melting point of the solids. The metalsubstructure, on the other hand, is rigid up to the melting points ofapproximately 2500° C., and significant cation mobility is not observedbelow ˜1300° C., although the oxygen in such compounds is highly mobileby 300° C. Indeed, lanthanide oxides are among the most thermally stablematerials known.

Materials with fluorite-based structures are “notorious” for the highmobility of the non-metal constituents. All LnO_(z) take at least onefluorite-based form, including fluorite, defect fluorite, pyrochlore,bixbyite, scheelite, inverse sphalerite, and many others. Thus, thesematerials are prime candidates for use in topotactic anion-exchangefabrication of single-crystal-like epitaxial, fiber-textured, andbiaxially textured films, and generally for the formation of oxide filmsof any texture.

Many examples of anion exchange involve a change in oxygenstoichiometry. Lanthanide oxides have been observed to transformtopotactically from one form to another as early as 1978, byhigh-resolution transmission electron microscopy. Other examples are inlanthanide-containing phases, such as the transformation ofperovskite-based LaNiO₃ to the metastable LaNiO₂.

d. Constrained Particle

Physical constraint has been demonstrated to hinder atomic rearrangementsufficiently to result in topotactic reduction of an oxide to a metal ina system for which topotactic anion exchange is not observed in thebulk.

A three-dimensionally constrained NiO fibrous intergrowth in ZrO₂ wasreported to reduce topotactically to Ni, the nickel cation lattice beingconstrained in three dimensions by the ZrO₂ lattice. The small size(layers ˜1000 nm in width) of the fibral intergrowths is believed tohave enabled stabilization of the nickel lattice by sufficientlyincreasing the kinetic barrier to cation reorganization. Such a reactionwould not be expected to occur for nickel that is not constrained inthis manner.

For thin films, and especially unrelaxed ultra-thin films, the physicalconstraint of epitaxy similarly restrains cationic rearrangement, but toa lesser extent. Epitaxial strain also shifts phase transitiontemperatures, and can thus stabilize phases not otherwise stable at roomtemperature. The constraint of epitaxy can stabilize LnO_(z) thin filmsystems, transformed from FCC cation-lattice LnX_(y) precursors, whichotherwise undergo a reconstructive transformation upon anion exchange,to yield high-quality LnO_(z) films.

3. Diffusional vs Reconstructive

Many topotactic reactions do not require atomic rearrangement of thecation and anion lattice at the reaction interface, and topotaxy doesnot imply a reaction mechanism, i.e., reconstructive topotacticreactions can occur. In some cases, significant rearrangement occurs,and the actual process can be viewed more as decomposition followed byreconstructive growth.

For the method of this disclosure, however, gross rearrangement of therigid FCC cation lattice does not occur. Thus, it is referred to as adiffusional topotactic reaction.

4. Bulk Lanthanide Systems

It has long been known that single crystals of certain lanthanide oxidescan be topotactically transformed from one oxide composition to anotherwith minimal change to the texture of the crystal. This is known tooccur readily for LnO_(z) C-type and fluorite-based crystal structures,which have an FCC-based cation sublattice, and particularly for PrO_(z)and CeO_(z). Single crystals of up to 1.45 mm in thickness are readilytransformed topotactically from z=2 to fluorite-based phases z=1.833 and1.67, respectively, and back to z=2 at about 350° C. The texturalquality of the single crystals is maintained. The utility of rapidoxygen exchange in LnO_(z) has been previously recognized, such as bulk(Ce,Pr)O_(z) which has been used as a reversible, active material foroxygen separation from air, with use in the temperature range of˜380-600° C. Yttrium metal, having an oxidized surface, has a greateroxidation resistance than any other elemental metal, due to itsprotective oxide scale which owes its adherency in part to near latticematch between metal and oxide.

Several LnO_(z) phases of different oxygen content can coexist in thesame single crystal, as demonstrated and known for TbO_(Z) and PrO_(s)in FIGS. 1, 2A, 2B, 2C, 2D and 2E. Some LnO_(z) such as CeO_(1.714),have been demonstrated to oxidize at room temperature, thusdemonstrating their high oxygen mobility, which in turn would imply thatthe orientational variants of homologous series LnO_(z) can renucleatenew domains at room temperature.

Although complete transformation of some pure LnO_(z) phases can resultin material failure, different thermal conditions or anion doping canhelp to stabilize low-oxygen-content compositions in an FCC cationlattice-based structure, or to stabilize a cubic fully-oxidized form.For example, although CeO₂ crystals transformed to CeO_(1.5) at 800° C.become polycrystalline, due to a reconstructive transformation to thenon-fluorite A-type phase, as is known, transformation below 600° C.results in the preferred stability of the C-type phase (see FIG. 3, aknown stability diagram for polymorphic forms of Ln₂O₃), which has anFCC cation sublattice and therefore remains single crystalline. Similarconditions apply for other LnO_(z) as well.

Nitrogen can stabilize the fluorite-type lattice of LnO_(z) at lowoxygen content. Single crystals of PrO₂ with a C-type structure can betopotactically reduced to fluorite-based PrO_(1.833) (the β-phase) by ahydrothermal technique using nitric acid as a solutant. All previousattempts to reduce A-type Pr₂O₃, in the absence of nitrogen, hadresulted in single crystals shattering, due to the reconstructive natureof transformation for pure oxide. Similar effects have been observed inattempts to reduce CeO₂ and ThO₂ single crystals.

Topotactic anion exchange is demonstrated also for somelanthanide-containing systems having a BCC cation sublattice. LaNiO₃, aperovskite, can be topotactically converted to LaNiO₂, through the useof annealing in the presence of solid hydrides such as NaH. The oxygensresiding between the lanthanum ions in the lattice are removed by theprocess. This is a true topotactic anion exchange process, but involvesonly changing the stoichiometry of the single anionic component of thesystem, oxygen. More complex anion exchange scenarios are possible.

5. Thin Film Systems

Topotactic anion exchange to produce heteroepitaxial barrier layers ofsome transition metals (Tr) is reported in the literature. Thetransformation in these materials is of poor quality, which isreasonable when they are considered from a crystal chemical perspective.It will be shown in later sections that lanthanides are most suitablefor topotactic anion exchange (Sec. 0).

Topotactic anion exchange from TiN to a TiO₂ barrier layer, has beendemonstrated and is known, but excessive porosity and roughnessassociated with the reconstructive conversion preclude its use in thismanner. Growth of epitaxial TiO₂ directly on silicon is difficult orimpossible, due to excessive interface oxidation during growth. A 120nm-thick TiN deposited by pulsed laser deposition was converted toepitaxial TiO₂ by exposure to 5 mTorr of oxygen at 780° C., yielding a(110) rutile TiO₂/(100) Si film with a two-domain morphology due toin-plane twinning. X-ray diffraction θ-2θ (FIG. 4) and rocking curvescans indicated an epitaxial film, with 1.7° FWHM texture for the TiNand 2.3° for the TiO₂. A TiO_(2 {)101} pole figure from the prior art,shown in FIG. 4, demonstrates that the TiO₂ is epitaxial. This film wasused as a barrier template for the subsequent growth of BaTiO₃. In asecond experiment, deposition of BaTiO₃ directly on TiN resulted in aBaTiO₃/TiO₂/TiN/Si epitaxial heterostructure. In both of thesedemonstrations, although the texture was acceptable, the conversion alsointroduced ˜70% porosity into the TiO₂ film, a completely unacceptableamount. Severe surface roughening was also observed. This is notsurprising, based on the near doubling of molar volume upontransformation, or a 19% isotropic linear lattice expansion. The TiO₂film had a high electrical leakage, indicating a high density ofelectronic and/or microstructural defects.

A second demonstration of topotactic anion exchange is that of epitaxialZr_(0.8)Y_(0.2)N to Zr_(0.8)Y_(0.2)O₂ on biaxially textured nickel andsingle crystal silicon. The reported textural quality of the resultantfilms is inadequate for applications. In one example from the prior art,epitaxial Zr_(0.8)Y_(0.2)N was deposited onto biaxially textured nickel(χ rocking curve FWHM=5.5°, φ FWHM=8.2°) by magnetron sputtering, andwas subsequently transformed to Zr_(0.8)Y_(0.2)O₂ using a water-forminggas mixture at temperatures from 500 to 900° C. A known x-raydiffraction analysis, FIG. 5, demonstrated a topotactic transformation.The texture of both the Zr_(0.8)Y_(0.2)N and Zr_(0.8)Y_(0.2)O₂ were 3.4and 4.8° in χ, and 10.5 and 11.0° in φ (FWHM), respectively, similar butslightly worse than that of the substrate (their texture is aconvolution of film and substrate texture), and inadequate forsuperconducting cable applications based on grain boundary misalignmentcriteria. In a second example from the prior art, epitaxial (Zr,Y)Nfilms grown on silicon were transformed in the same manner to (Zr,Y)O₂.These were demonstrated by cross-sectional transmission electronmicroscopy and diffraction analysis to have a topotactic orientationrelationship at the conversion front, shown in the art at FIGS. 6A, 6B,6C and 6D, but their overall texture was poor.

The yttrium-doped zirconium oxide topotactic anion exchange system,Zr_(0.8)Y_(0.2)O₂—Zr_(0.8)Y_(0.2)N, possesses the appropriatecrystalline structure for successful topotactic conversion, i.e., arocksalt precursor and a fluorite product, both having an FCC cationsublattice. But, the 50% volume change upon anion exchange (13%difference in lattice parameter) is too large to allow for conversionwithout the introduction of undesirable mosaic structure. Indeed, filmsin this system crack and spall due to compressive strain.

Some metals are known to form a thin topotactic film on their surface asthey oxidize. If it is possible to deposit epitaxial films of lanthanidemetals on desired substrates, and if the change in lattice dimension issmall enough upon conversion from Ln to LnO_(z), then this method couldalso be technologically useful.

Although topotactic oxygen loss or gain is demonstrated to have limitsin single crystals, it is likely that epitaxial thin films will exhibitan extended range of stability for such reactions due to a combinationof the physical constraint of epitaxy and the very low thickness of suchfilms.

6. Microstructure and Texture Issues

The preceding sections show some examples of texture evolution throughtopotactic anion exchange. It is instructive to summarize the texturaltrends before moving on to a full description of the crystal andchemical systems for which topotactic anion exchange is a viable methodof fabrication for epitaxial or textured films.

A common predictor for determining the likelihood of “good-quality”heteroepitaxy in a given oxide system is lattice match. Other factors,such as chemical compatibility, contribute, and this predictor fails incertain cases (for example for the epitaxial growth of lanthanide oxideson (100) silicon, which grow with a (110) orientation, in contrast to alattice-match prediction of (100) growth) but it is largely accurate,and is suggested to be the primary parameter to use in down-selecting anappropriate composition from those that are compatible with the systemunder consideration.

The predictor of lattice match is even more effective in predicting thetextural “quality” of topotactic anion-exchange systems. Many lanthanideoxide systems have very low changes in lattice dimension uponconversion, making lanthanides the ideal system for successfulapplication of this method. Furthermore, the wide range of cation andanion alloying possible for lanthanides enables further minimization ofconversion strain effects (see Sec. I.).

The general microstructural trends for the effects of topotactic misfitare summarized in Table I. Small elastic strains are relaxed by theintroduction of dislocations, which cause mosaic texture. Subsequentanion exchanges may not introduce any additional mosaic, due to theexisting mosaic structure relieving such stresses. Likewise, a film withsome degree of mosaic texture present from film growth may not show anincrease in mosaic texture upon conversion. In many reversiblereactions, this mosaic is generated upon first conversion, and does notchange with further cycling. Product film texture is a convolution ofall preceding textures.

Texture is further affected by anisotropy of precursor, product, or bothphases. Insufficient demonstrative examples exist, but the effects canbe predicted based on crystal chemical and epitaxial growth principles.Orientational variants arise to minimize strain energy. For example, twodifferent topotactic orientations are commonly observed in the NiO—Al₂O₃system.

TABLE I Microstructural effects of lattice mismatch for topotactic anionexchange systems. Condition Effects tension - high porosity crackingincomplete exchange roughness tension - low mosaic texture perfect matchnone compression - low mosaic texture compression - high porosityspallation pulverization of material roughness

An example of high compression upon topotactic conversion is thereaction of Ta₂O₅ to form TaON and Ta₃N₅, which results in the formationof a large network of pores, due to the large volume change ontransformation.

In a system with moderate compression, Mg₂SnO₄ films formed from SnOvapor and a MgO film take on a four-fold mosaic texture, with acharacteristic network of dislocations form at the interface and domainstilted approximately 0.8° from the MgO <100>, due to a 2.5% latticedimension mismatch between parent and product phases. And likewise withMg₂TiO₄ formed from the reaction of MgO and TiO₂, characterized by theformation of numerous cation antiphase boundaries. This transformationis reconstructive.

A system with moderate tension, but with a diffusional topotacticreaction, reduction of CeO_(z) has a lattice expansion of ˜2.3%, andforms coherent domains of ˜40 to 400 nm in size. Upon reoxidation, themosaic disappears, demonstrating that the reaction is diffusional andnot reconstructive.

The high tension of dehydration of Ni(OH)₂ to NiO and of Mg(OH)₂ andCd(OH)₂ are examples of systems which become porous upon topotactictransformation. A second example is the crystal surface of MgO—Nb₂O₅,which results in excessive roughness.

F. Crystal Systems for Topotactic Anion Exchange

The requirements for films to be fabricated by the topotactic anionexchange method are that they consist of a rigid cation sublattice, andthat they have high anion diffusivities at temperatures which the filmand substrate can tolerate (some degree of atom motion occurs at alltemperatures, but among epitaxial oxide template materials, those belowpossess the qualities to the highest degree), thus enabling aniondiffusion and exchange while maintaining the orientation and texture ofthe parent crystal. These types of materials have been known for sometime, but have never been applied to thin film technology. Thedescription by Bevan and Summerville (1979) is quite clear:

-   -   “There is a host of solid phases of widely differing chemical        composition for which a common feature is the stable arrangement        of cations, either ordered or disordered, on a sub-lattice which        is essentially that of the cations of the fluorite-type        structure. The stability of this cation arrangement is then such        that the anions of the associated anion sub-lattice can adopt a        variety of configurations around individual cations, each of        which is the most appropriate for the particular cation/anion        ratio combination . . . . Moreover the anion sub-lattices thus        produced are clearly very flexible and can adjust within wide        limits to changes in cation composition and distribution so long        as the cation sub-lattice per se retains its relationship to        that of the fluorite-type structure, and gross non-stoichiometry        in fluorite-type phases is a direct consequence of this.”

This “host of solid phases” consists primarily of crystal structuresbased on a close-packed face-centered-cubic (FCC) cation sublattice. AnFCC cation sublattice is also present in rocksalt-type structures,however, with anions occupying different sites. Other cationarrangements can also exhibit this behavior to a limited extent, forexample those with a close-packed hexagonal cation sublattice, or thosewith a body-centered cubic (BCC) cation sublattice.

1. Face-centered Cubic Cation Sublattice

a. Simple Lanthanide Phases With an FCC Cation Sublattice

All LnO_(z) exist in several crystalline forms, most of which consist ofa rigid FCC cation sublattice, with oxygens distributed in variousmanners. These phases exhibit high oxygen mobilities, and it is thesephases that can be synthesized through diffusional topotactic anionexchange. Such phases can also be inter-transformed by the same method,dependent upon possible ionization states for a given lanthanon.

The crystal structural similarity of FCC cation-sublattice-based LnO_(z)phases is shown in FIG. 7. dark circles represent the lanthanidecations. The unit cells of these phases are all FCC. Oxygens inrocksalt-type LnO_(z) lie in octahedral sites, and in fluorite-typeoxygens lie in tetrahedral sites. The C-type LnO_(1.5) can be consideredto be derived from fluorite through the removal of ¼ oxygens. Due to thelarge cationic radii of the lanthanides, the lattice parameters for thepossible LnO_(z) phases for any given lanthanon vary from 1.8 to 5.5%.The rigid FCC cation lattice defines the structure, unlike typicaltransition metal oxides. All three of these LnO_(z) are epitaxially andtopotactically compatible. This is a moderate mismatch, and results inthe generation of nanodomains with minor mosaic upon topotacticconversion. It will, however, be shown below that this lattice mismatchcan be reduced substantially, so that in practice essentially perfecttopotactic conversion is possible.

Rocksalt-structured phases of pure lanthanide monoxides exist only forEu and Yb. Others have typically been found to be nitrogen orcarbon-stabilized. All lanthanide sesquioxides can assume the C-typebixbyite, essentially a fluorite structure with ¼ of the oxygensremoved, and one half of cation positions distorted by ˜12% from FCC.From Nd₂O₃ to La₂O₃, they may be metastable, but are observed perhapsdue to low oxygen diffusivities at those temperatures.

Rocksalt-based, fluorite-based, and mixed lanthanide salts (LnX_(y))also exist. These include LnH₂, LnH₃, LnC_(0.33), LnN, LnP, LnS, LnSe,and LnTe, and have wide anion solubility ranges, as demonstrable byperusal of known compounds in materials databases, such as the PowderDiffraction File or the Landolt-Bornstein Numerical Data and FunctionalRelationships in Science and Technology. Superstoichiometric phaseshaving oxygen in all octahedral sites, and some interstitials, are alsoknown. Cations in CeD_(2.29), LaD_(2.30), and PrD_(2.37) (D=²H) aredisplaced slightly from the ideal fluorite, but not enough to formsuperlattice reflections in XRD patterns. In other words, very slightly,such that a good epitaxial fit is maintained.

“Rocksalt-based” indicates crystal structures comprised of a nominallyFCC cation sublattice, with an interpenetrating nominally FCC anionsublattice, typically having a wide range of anion:cationstoichiometries, from about 0.2:1 to 3:1. Anions lie in octahedralsites.

“Fluorite-based” indicates crystal structures comprised of a nominallyFCC cation sublattice, with an interpenetrating nominally ½a₀ cubicanion sublattice, typically having a wide range of anion:cationstoichiometries, from about 1.2:1 to 2.5:1. Anions lie primarily intetrahedral sites. The structure of C-type Ln₂O₃ (bixbyite) isconsidered fluorite-based.

Alloys also exist, for example lanthanide oxide-fluorides have beenknown since 1941, with the description of La(O,F)_(y). The materialaccommodates an excess of anions as interstitials in a fluorite-typecrystal structures. The anions in these phases are highly mobile, as isthe case with other fluorites. (In many descriptions of chemicalformulae of alloys, subscripts of individual alloy constituents areomitted, as many of these exhibit wide, continuously variable ranges ofcomposition.)

Not all LnO_(z) phases are based on FCC cation sublattice. Because theradius ratios of LnO_(z) are close to critical values, several otherstructures are observed. For LnO_(1.5), A-type (hexagonal) and B-typephases can be considered to be derived from a fluorite-type parent, withthe oxygen lattice preserved and the cation sublattice sheared to form alayer-type of structure, as known and illustrated in the art in FIG. 8.Such a crystal structure is not the most amenable to the topotacticanion exchange method, but in certain cases, such as with mixed high andlow-diffusivity anions, is applicable. Epitaxial stabilization andkinetic limitations may act to stabilize the C-type form of some LnO_(z)that would otherwise exist in the A or B-type form. All lanthanideoxides are stable (or metastable) at room temperature in the cubic form.At temperatures up to 1000° C., some undergo phase transitions to otherforms, but the distortion is slight enough that epitaxy could stabilizethe fully cubic form, or that the distortions would be low enough toavoid the introduction of domain boundaries in such a film.

b. Ordered Anion Phases

In some lanthanide oxides, many phases of composition intermediatebetween Ln₂O₃ and LnO₂ exist. The many different phases are members of ahomologous series, Ln_(m)O_(2m-m), first described for thePr_(n)O_(2n-2) system in 1965 (more specifically, the Ln_(m)O_(2n-2m)homologous series). The FCC cation sublattice is essentially identicalfor all of them, such that the differences in structure are due tovarious arrangements of oxygen within this rigid FCC cation lattice. Thedifferent homologues form by the ordered introduction or removal ofoxygen vacancies, with no cation interstitials required for thetransformation.

These many phases have a wide variety of monoclinic, triclinic, andrhombohedral unit cells that appear deceptively different from oneanother. They are all, in fact, essentially an FCC cation sublattice,each with a different ordering of the anions, and require no diffusionalmovement of the cations. This is shown schematically in the art in FIG.9, which shows the Ln cations as black dots, viewed along [110] of theFCC cation sublattice. The unit cells for several different m areoverlain this lattice, demonstrating the similarity they all share.

The various possible anion excess stoichiometries result in numerousdifferent crystalline phases, called vernier phases. The structures ofthese phases are all derived from the same parent prototype structure,and differ only in anion stoichiometry, anion site ordering, and slightdisplacements of the cations, and slight distortions from the prototypecubic unit cell.

A great many composite modulated structures are known for anion alloysas well, such as Ln-O—F, Ln-O—N, and Ln-N—F systems. These are all basedupon the fluorite structure, and differ only in the ordering of theanions through the rigid FCC cation lattice. For example, in the systemY—O—F, distinct phases exist for Y₅O₄F₇, Y₆O₅F₈, Y₇O₆F₉, Y₁₇O₁₄F₂₃, andso on. All of these have the same rigid FCC cation lattice, and differin their cube-side-equivalent unit cell dimension by less than 0.4%.This is a very small mismatch, and indeed these phases intergrowreadily. They consist only of ordering of the anions, due to highmobility of the anions.

Anion exchange is a continuous process, as evidenced by the smoothchange in lattice parameter with changing anion stoichiometry, having noabrupt, discrete shifts, which would tend to introduce defects, as shownin the art in FIG. 10. Changing anion stoichiometry does not necessitateany reordering of the cations, and thus topotactic oxygen exchange isnot disruptive to an epitaxial film or single crystal. The reactionsused to form various compositional variants of lanthanide oxides involveonly a reaction with gaseous oxygen at elevated temperature.

The structural compatibility of LnO_(z) phases is demonstrated by theirtopotactic coexistence in a single crystal, as observed byhigh-resolution transmission electron microscopy (HRTEM). An example isshown for the PrO_(z) system in the art in FIG. 2, a HRTEM image ofthree domains in a single crystal of PrO_(1.8). These domains havemonoclinic, triclinic, and an undetermined unit cell, all with the sameparent FCC cation sublattice, and have compositions of PrO_(1.833),PrO_(1.778), and PrO_(1.833), respectively. Topotaxy is demonstrated bythe electron diffraction patterns from the different regions. A secondexample is shown in the art in FIG. 1, where six different homologuesare present in a single crystalline region of TbO_(z) in an area lessthan 30×15 nm².

The situation of topotactic phase boundaries (or contiguousmicrodomains) of different compositions and structures is commonlyencountered in materials involving reduced lanthanide oxides. It isimportant to note that macroscopic techniques such as powder x-raydiffraction are insensitive to these multiphasic microdomains.

c. Ordered Cation Phases

Many lanthanide-containing oxides having ordered cation arrays exist,and synthesis of these phases is amenable to the topotactic anionexchange method. Pyrochlore phases are essentially fluorite phases, withordering of cations of differing valence on different sites in theFCC-based cation sublattice. Order tends to decrease anion diffusivitiesfor a given composition that forms fluorite-type and ordered pyrochlorephases.

Crystalline phases comprising a major fraction of lanthanides are alsosuitable for topotactic anion exchange. As for the pure lanthanidecases, it is preferable but not necessary for both the precursor andproduct phase to have an FCC cation sublattice. An example of one suchfamily of systems, Ln₂B₂O₇ pyrochlore phases. Pyrochlore is afluorite-type phase, with A and B ordered in octahedral and cubiccoordinated lattice sites, and with ⅛ of the oxygens removed. Numerouspyrochlore phases have excellent lattice match with common epitaxysubstrates, as shown in FIG. 11. Some of these phases exist in knownLn₂B₂X_(m) phases, but the literature record is incomplete. It ispossible to predict, on the basis of oxygen mobilities in the pyrochlorephases, and on the existence of LnX_(y) phases, which precursors arelikely to exist, and which will best convert favorably to the desiredoxide pyrochlore. (It should be noted that similar structures having thesame or similar compositions also exist, in the form of numerousfluorite- and pyrochlore-related phases. These phases are nominallyidentical but with the A and B randomly distributed among the FCCsites.)

Ordered phases of alloys that are not entirely lanthanide-based, such asLnTrO₄ Scheelite type phases (Tr=transition metal), observed for Ndthrough Ho, are also suitable. The cations are alternately layered along<110> of a fluorite-type parent to yield a tetragonal unit cell that isessentially a superstructure of fluorite. An example in the art is shownin FIG. 12, for EuWO₄. An epitaxial precursor to a scheelite-type oxidewould preferably be c-axis oriented, or less preferably with the c-axisin-plane, such that a domain structure results. Other orientations canalso be useful. Distorted variants such as monoclinic Fergusonite mayalso be synthesized by the method. Zircon and monazite are highlydistorted variants of scheelite, and are unlikely to easily yieldtopotactically converted oxide films without the introduction ofsignificant defects. Other series of cation-ordered and fluorite-basedlanthanide oxides are those with the wolframite structure. A diagram ofthe observed oxide phases for several LnTrO₄ is known and shown in FIG.13.

For any of these oxide phases having ordered cation arrays to be formedby the topotactic anion exchange method, a suitable precursor having thesame cation ordering is necessary. The ternary, quaternary, and highersystems that would yield such phases have not been fully explored. It isexpected that such phases can be synthesized under the appropriateconditions, especially with appropriate selection of anions (Sec. I.1).

d. Layered-structure Materials

Layered phases can be used in a straightforward manner to present thedesired surface. For example, a layered phase having a nominally squarea-b, but a long c axis can be grown in a c-axis orientation on asubstrate, and subsequently converted. Changes in the c dimension uponconversion are typically irrelevant to the process, because it ischanges in the dimension of the a-b plane through conversion that areimportant for preservation of a functioning template layer. Anisotropyof candidate phases are both permanent and transitional.

Some permanent layered phases are the Ruddlesden-Popper series ofphases, A_(n+1)B_(n)O_(3n+1), based upon n number of perovskite-likesubcells interleaved with rock-salt-type oxide double layers. Theprototype structure of the n=1 compositions is K₂NiF₄. Numerouscombinations of lanthanide oxides and other cationic species withoxygen, nitrogen, fluorine, and other anions are known. The rigidity ofthe lanthanide component of the cation lattice is sufficient in manycases to yield a topotactically transformable system. Aniondiffusivities in many of these systems are unknown, but can be estimatedbased on their composition and structural families. Additionally,Ruddlesden-Popper phases have a BCC-based cation sublattice (see Sec.0).

Transitional layered phases are those for which the precursor is highlyanisotropic, but the product phase more closely approximates a cubicphase. An example of this is LnC₂ phases, which have an a-b mesh thatclosely matches that of analogous Ln₂O₃, but for which the c axis isexpanded by about 65%. A c-axis LnC₂ film will convert topotactically toLnO_(z) via evaporation of C in the form of CO, with a deflation in thec dimension of the lattice as the conversion front proceeds. LnC₂ havea-b plane lattice parameters, (Å), which closely match those formaterials of technological importance, as shown in Table II.

TABLE II Basal lattice parameter (a0) of LnC2 phases (Å). ScC₂ YC₂ LaC₂CeC₂ PrC₂ NdC₂ SmC₂ GdC₂ a₀ 3.99 3.644 3.92 3.88 3.85 3.82 3.76 3.718√2a₀ 5.64 5.18 5.54 5.49 5.44 5.40 5.32 5.26 TbC₂ DyC₂ HoC₂ ErC₂ TmC₂YbC₂ LuC₂ a₀ 3.690 3.669 3.643 3.620 3.600 3.637 3.563 √2a₀ 5.22 5.195.15 5.12 5.09 5.14 5.04

e. Stuffed Structures

Stuffed structures, those with double layers of anions, are not amenableto the topotactic anion exchange method. In these phases, the lattice isdefined to a great extent by the anion lattice. Thus, although atopotactic transformation is possible, it will be reorganizational.Examples of such systems include LnCl, LnBr, Ln₂N₂S, and Ln₂O₂S. ZrCland ZrBr have graphite-like layered structures and take up oxygeninterstitials easily.

f. Silicates

Some silicates exist, having crystal structures derived a distorted FCClattice, and are likely to be amenable to the topotactic anion exchangemethod. Moreover, such phases are stable in contact with silicon.Appropriate Ln-Si—X phases remain to be identified. An example isolivine, M₂SiO₄. Silicon atoms are present in an alternating fashionbetween half of the neighboring M cations, such that the result is anorthorhombic phase, for which the oxygen lattice presents a squarelattice on (011) and equivalent surfaces. This square lattice matches upnominally with that of Ln₂O₃ and perovskite phases.

2. Body-Centered Cubic Cation Sublattice

The extreme resistance of lanthanide cations to diffusional migration inoxide LnX crystalline phases allows extension of the method to otherstructures that are not based on an FCC cation sublattice, and that havelanthanides as a major cationic component, namely ones having abody-centered cubic (BCC)-based cation sublattice.

It is possible to transform some perovskite-like (ABO₃) precursors tooxide perovskite-based phases. In these, the cation lattice is BCC, ortwo interpenetrating simple cubic cation sublattices. The cations can beall lanthanides, in the case of DyScO₃, can have one component be alanthanide, LaAlO₃, or either of these can be alloyed to some degreewith other cationic species of either lanthanide or other metals, suchas transition metals or alkaline earth metals. Many of these phases areslightly distorted from cubic. Although most known lanthanide-containingperovskites are oxides (based on the strongly covalently bonded BO₆ ²⁻octahedra), some non-oxides and other partial oxides are known to exist.

Recent work has shown that perovskite-type LnBO₃ may have extremely highpotential as barrier layers. Anion diffusivities in ABO₃ are typicallymuch lower than for FCC-based phases. In fact, it has been demonstratedrecently that single layers of oxygen vacancies can be trapped duringgrowth of epitaxial SrTiO₃, at 760° C. An implication of this is that alanthanide-containing perovskite-based (BCC-type cation sublattice)precursor only a few atoms thick can be sufficient to act as a barrierto oxygen diffusion to a sensitive substrate such as silicon, when onlythe top one or two atom layers is converted to oxide due to kineticconstraints. This is sufficient for templating action of the epitaxialbarrier layer as well.

G. Lattice Match

1. Epitaxial Quality

For epitaxial growth of highly ionic materials such as oxides, ifstructure and bonding at the interface are compatible, lattice matchwith the substrate is a major determinant of the quality of epitaxialfilm that can be grown. Thus, after chemical stability, lattice match isthe main selection criterion useful for down-selecting the number ofcandidate systems for production of an epitaxial template layer viatopotactic anion exchange.

Good lattice match minimizes defects. Misfit strain induces theformation of misfit dislocations, leading to mosaic spread in anepitaxial film. A high degree of misfit can lead to strain-inducedsurface roughening.

The lattice parameter of the pure lanthanide oxides varies with oxygencontent, as demonstrated in FIG. 14, a plot of the pseudocubic FCCcation sublattice dimension versus oxygen content for the lanthanides.The horizontal axis represents the cation radii, and all of thelanthanides are marked on that axis. All lanthanides are observed asLn₂O₃ bixbyite (C-type) at room temperature. Under oxidizing or reducingconditions, some will form higher or lower oxides, such asrocksalt-structured EuO and YbO, or fluorite-type NdO₂, CeO₂, PrO₂, orTbO₂. Several intermediate phases in a homologous series lie betweenLn₂O₃ and LnO₂ for the latter, consisting of variations in the orderingof the oxygens in the FCC cation sublattice. These phases readilytransform topotactically from one homologous series member to another.The retention of crystalline integrity with changing oxidation statesindicates the suitability of these phases for the topotactic conversionmethod.

To maximize the quality of film epitaxy, for both the LnO_(z) and forany epitaxial film which will be deposited on the LnO_(z) templatelayer, it is necessary to select a LnX_(y) composition that has asuitable lattice match with the substrate (and for LnO_(z) to thetemplated film). The lattice parameter of a large number of LnX_(y) areshown in FIG. 15, along with the lattice parameter of some commonsubstrates used for epitaxial deposition, for comparison. Each datapoint indicates a specific lanthanon and a specific anion for an LnX_(y)(e.g., the filled diamond at the bottom left represents YbN, witha₀=4.81 Å). All of these phases have crystalline structures based on anFCC cation sublattice, although a few of them take hexagonal form atelevated temperatures, which epitaxial strain can stabilize the C-typeform. Many LnO_(z) have been reported and later shown to benitrogen-stabilized rocksalt-structured Ln(N,O), or occasionallyhydrogen, carbon, or sulfur stabilized. These are not included in theplot due to uncertainties in reported values available in theliterature, but they are certainly viable candidates for the method. Aninfinite variety of compositions exists by cation substitution, cationdoping, cation alloying, anion substitution, anion doping, anionalloying, and partial or excess anion site filling, such that it is notpossible to enumerate all of the possibilities.

Once several compositions are selected for potential use with aparticular substrate-film system, it is useful to further consider themagnitude of the change in lattice parameter upon conversion to oxide.In general, the greater the change in lattice parameter from LnX_(y) toLnO_(z), the greater will be the mosaic spread introduced into the filmtexture upon conversion. Additionally, for more severe lattice dimensionchanges, defects such as cracks, domain boundaries, trapped vacancies,or even spallation of the film can occur. Thus, it is most reasonable toselect a system having the smallest possible change in lattice parameterupon conversion, while taking into consideration all other potentialfactors as well such as chemical compatibility, electrical behavior,etc. To aid in selection, the absolute change in the lattice parameter(Å) of the pseudocubic subcell for a large number of LnX_(y) are shownin FIG. 16. An average value for the pseudocubic lattice parameter isused for those phases with slightly differing parameters for thepseudocubic a, b, and c axes, such as those with a slight tetragonaldistortion. Depending upon the lattice dimension of the substrate, thesefilms may have domain structures and therefore the actual change inlattice parameter may vary slightly, on the order of a few tenths of apercent, variation which in any case is to be expected for processesinvolving different deposition conditions, film qualities, etc. Theanion stoichiometry can be adjusted so that the change in latticeparameter is zero, for example by an appropriate mixture of N and F, Nand S, or perhaps N, F, and S together, with or without the addition ofoxygen, as may be allowed for a given chemical system.

It is further useful to consider the relative change in the latticeparameter of the pseudocubic subcell with conversion. This data is shownin FIG. 17. A smaller change is preferred, but this will not always bepossible. A general rule of thumb is to keep this misfit below 5% tominimize the introduction of defects.

The process is also applicable for biaxially textured, fiber textured,and polycrystalline films.

2. Strain Affects Lattice Parameter

Bulk and single crystals of lanthanide oxides undergo polymorphictransformations, and frequently shatter on cooling from very hightemperatures, particularly for displacive transformations. Such problemsare mitigated or absent in epitaxial films, which are very thin in onedimension, and are clamped at the film/substrate interface.

3. Epitaxial Stabilization and Topotactic Stabilization

The power of topotactic stabilization likely exceeds that of epitaxialstabilization. In epitaxial stabilization, tensile or compressive strainenergy tips the energy balance to stabilize one phase in preference toanother, when thermochemical conditions for an unstrained material wouldotherwise favor only one.

The lattice constraint of epitaxy allows a fine degree of control overthe energetics of phase formation, because it allows the control ofstrain energy, which is not a possible mode of control in bulk systems.This can allow the formation of materials impossible to synthesize byconventional means. Because material undergoing a topotactic reaction isconstrained in three dimensions, not the two dimensions of a surface,cases exist wherein a greater strain can be tolerated before strainrelief mechanisms begin to act. Moreover, the reduced dimensionality ofthe thin film form can reduce the build-up of three-dimensional strainwhich causes mosaic or disintegration of three-dimensional singlecrystals.

For example, the topotactic dehydration of MO₃.⅓H₂O, M=W or Mo, yieldshexagonal MO₃, which cannot be formed by conventional methods. Thetransformation is topotactic and transformational, progressing byoriented nucleation and growth to yield a product with an orientationrelationship of (001)_(parent)∥(001)_(product) and[100]_(parent)∥[100]_(product). Dehydration of MoO₃.⅓H₂O yields ametastable monoclinic MoO₃ with a distorted ReO₃ structure. Thistransforms irreversibly at ˜370° C. to the stable orthorhombic form.

4. Thermal Expansion

In choosing a film to obtain optimal lattice match, it is important toconsider the coefficient of thermal expansion of the substrate, and ofthe precursor and product films. Rare earth sesquioxides have thermalexpansion coefficients ranging from 8.3 to 10.8 ppm/° C., overapproximately room temperature to 800° C. Because this parameter doesnot vary much between systems, it is considered a minor consideration.

H. Cation Selection

Lanthanides having similar valence behave similarly. These are shown inTable III. LnO_(z) with multiple valence are considered most amenable totopotactic anion exchange. On the other hand, LnO_(z) with only a singlevalence, three, will be the least likely to have high concentrations ofelectronically active defects, but these can be pacified to some extentby hydrogen anneal, after complete conversion.

TABLE III Observed valence states of lanthanide elements. Valence StatesLanthanide 2, 3 3 3, 4 4 Sc Sc Y Y Zr Zr La La Ce Ce Pr Pr Nd Nd Pm PmSm Sm Eu Eu Gd Gd Tb Tb Dy Dy Ho Ho Er Er Tm Tm Yb Yb Lu Lu Hf Hf

I. Anion Selection

1. Lattice Sizes and Chemistry

Smaller anions, Row II of the periodic table, are most amenable to themethod. Hydrogen has been observed to stabilize many rocksalt LnX_(y),through incorporation into the tetrahedral sites. It is also possible tofill half of the tetrahedral voids with transition elements such as Niand Pt. These are sometimes referred to as filled-up rocksalt-typephases, and are candidates for the topotactic anion exchange process.Diffusion of anionic components would be expected to be greatly slowed,with intermediate diffusivities on partial filling-up.

Ionic radius and charge must be considered in selecting appropriateanions. Preferred anion constituencies are those that most closelyapproximate both the charge and ionic radius of oxygen, as an average ofthe constituents. For example, a 50:50 mixture of N³⁺ and F⁻ is highlypreferred, as their charges average to −2 (that of oxygen), theiraverage ionic radius matches that of oxygen, and their individual ionicradii differ from that of oxygen by only 4%. Furthermore, becauselanthanide phases based on an FCC cation sublattice (and alsoperovskites) can accommodate appreciable deficiency or excess of anioniccomponents, the ratio can adjusted appreciably, and optimizedexperimentally. Another highly preferred anion constituency is onecontaining sulfur. The charge and ionic radius closely match that ofoxygen. Sulfur's covalent bonding characteristics do differ from thoseof oxygen, so it cannot be substituted for oxygen without regard to thisdifference, as anion-rich lanthanide oxysulfides tend to have layeredstructures. Here, it is important to note the power of topotacticstabilization and kinetic limitations, such that a Ln-S metal-nonmetalcompound may be successfully converted to a Ln-O metal-oxide compoundand retain its FCC cation sublattice, as the temperature for topotacticanion exchange is far below that necessary for appreciable cationicrearrangement. Also, the interface between the Ln-S and Ln-O, whereconversion is occurring, is constrained in three dimensions, placingfurther energetic limitations on the formation of phases which would bethermodynamically preferred in an unconstrained state. It is expectedthat these principles can be applied by one skilled in the art todetermine further anion constituencies that will also approximate oxide,such as combinations of N, F, and S.

An infinite variety of anion stoichiometries exists, such that it is notpossible to enumerate all possibilities. The nonmetal constituents ofthe metal-nonmetal film are preferably chosen from the group consistingof: H, C, N, F, P, S, Cl, Se, Br, Te, and combinations thereof. Thenonmetal constituents of the metal-nonmetal film are more preferablychosen from the group consisting of: H, C, N, F, P, S, Cl, andcombinations thereof. The nonmetal constituents of the metal-nonmetalfilm are most preferably chosen from the group consisting of: N, F, S,and combinations thereof.

2. Exchange Mechanisms (C-->CO)

It is important to consider the chemical species involved in anionexchange for the various systems, very large vapor species are prone topore formation. For oxides, nitrides, and fluorides, exchange is byionic diffusion to the surface and evaporation of diatomic elementalspecies. This may also be the case for phosphides, chlorides, selenides,and tellurides. For anion exchanges involving sulfur, formation of SO₄²⁻ can be suppressed or reversed by ammonia gas. For anion replacementof carbon by oxygen, carbon is in some cases evolved as CO, dependentupon oxygen activity.

3. Partial Oxides

Compositions intermediate between LnX_(y) and LnO_(z) are also candidatecompositions for the precursor. A partial oxide, Ln(O,X)_(y), isdeposited at a lower oxygen partial pressure than the pure oxide, suchthat the substrate is not oxidized, and is then topotactically convertedto the full oxide. For example, if Ln₂O₃ cannot be deposited on asubstrate without substrate oxidation, it may be possible to depositLn₂(N,F,O)_(y), where the N:F:O stoichiometries vary with depositionconditions (gas background pressures), and which would require a loweroxygen partial pressure during growth, perhaps low enough to avoidsubstrate oxidation. For a given system, there will exist a maximumoxygen content (or deposition process p_(O2), H₂/H₂O ratio, etc.) atwhich substrate surface oxidation will be prevented. Specific parametersmust be evaluated for each case. Likewise, post-deposition conversion toa full oxide is not always necessary, depending upon the specifics of aparticular application and system. In general, oxygen content inprecursor films is preferably 0 to 80%, more preferably 0 to 50%, andmost preferably 0 to 20%.

4. Multistep Processes

An important consideration for a topotactic anion exchange process isthat an anion stoichiometry exchange pathway that must allow theintegrity of the single crystal to remain. For example, although directreduction of CeO₂ for to CeO_(1.67) results in a reversible topotacticreaction, more extensive reduction to CeO_(1.5) (Ce₂O₃) results indisintegration of the single crystal, and the formation of apolycrystalline product. Ce₂O₃ assumes the A-type form at thetemperatures of oxygen anion exchange used for that experiment, so thisis not an unexpected result. The same type of exchange for a LnO_(z)which assumes the C-type or fluorite-type structure at the exchangetemperature is considered more likely to undergo the exchangesuccessfully. Epitaxial constraint could also play a role in stabilizinga form having an FCC cation sublattice. Alternatively, it is possiblethat a pathway involving other anions would allow the FCC cation latticeto be maintained through the entire transition. Perhaps nitrogen offluorine. There is a minimum concentration of various anions requiredfor maintenance of the rigid cation sublattice. These can be determinedfrom phase diagrams, and may be interpolated from the phase diagrams foranion alloys that have not been experimentally investigated.

J. Alloying

1. Cation

Wide intersolubility is observed for LnX_(i), solid solutions of two ormore differing Ln cations. This can be used to advantage to tailor thelattice parameter for a given system. It is also useful for controllingelectronic and other properties of the films. Those with more similarradii are more likely to form continuous solid solutions across therange of compositions. An infinite variety of cation stoichiometriesexists, such that it is not possible to enumerate all possibilities. Thefollowing provides guidance in selecting an appropriate stoichiometry.

a. Other Ln

Fluorite and bixbyite-structured oxides have wide solid solution ranges,and correspondingly widely varying lattice parameters and aretopotactically compatible. Extensive solid solubilities of LnO₂-Ln′₂O₃systems are observed to have fluorite or C-type structures, and todisplay a widely variable range of lattice parameters. The latticeparameter of solid solutions of CeO₂ with several Ln′₂O₃ are shown inthe art in FIG. 18. These cover a range of 0.4 Å, with some systemscovering a range of 0.2 Å between CeO₂ and Ln′₂O₃. It is important tonote that, over almost the entire range, either fluorite or C-type oxideis the observed phase, indicating topotactic compatibility over thiswide range. For systems that have not been measured, the latticeparameter of mixed oxides (A_(x)B_(2-x)O₃) having the same structure canbe predicted using Vegard's law, which predicts a linear relationshipwith x. Similar trends are observed for LnX_(y).

b. Other Metals

Other dopants, alloy components, and additives that might be added to anoxide can be incorporated in the precursor composition. For example,scandium can be added to ZrN, to yield scandium-stabilized cubiczirconia, which has the highest anion conductivity of all single dopantZrO₂ systems. The lattice parameter of three ZrO₂-Ln₂O₃ cation alloysystems are shown in the art in FIG. 19. Note that either fluorite,C-type, or delta (fluorite-type) are observed over the entirecomposition range, indicating complete topotactic compatibility.

Numerous other ternary and higher lanthanide-containing crystallinephases are known to have structures having an FCC cation sublattice.These are also candidates for topotactic anion exchange. As halide unitcells increase in size through alloying, (Ca,Ln)F₂, unit cell volumesincrease linearly, and also defect concentrations increase. Aliovalentcation dopants can also be used to alter electronic and atomicmobilities.

Examples of other functionality that can be provided by doping andalloying are as follows. Alloying can be used to minimize the depositiontemperature required for a film, especially in cases where alow-melting-temperature substrate is used. Aliovalent doping can be usedto alter the electrical conductivity of product films, and also toaffect the anion mobilities in both precursor and product. This may bedone to decrease mobilities, as in the case of ultra-thin films, or toincrease mobilities, as in cases where the product phase will be used asan electrode. Generally, cation alloying is preferably 0 to 55%, morepreferably 0 to 30%, and most preferably 2 to 20%. Cation alloying withGroup IVA metals (Ti, Zr, and Hf) is preferably 0 to 75%, morepreferably 0 to 49%, and most preferably 0 to 25%.

Compatible phases are not limited to those described. The method isextensible to include related structures. For example, structuralcompatibility of alkaline earth elements with lanthanide elementsindicates that phases such as BaZrN², K₂NiF₄-type compounds, LiTiO₂(close-packed anions, rocksalt-type structure), and other related phasescan by synthesized from a LnX precursor. The incorporation of othercations are also possible, as long as the integrity of the FCClanthanide cation sublattice is maintained. Manners in which the FCCcation sublattice could be broken are, for example, the insertion of anextra layer of alkaline earth ions every third cation layer, or thereplacement of some of the lanthanide cations in their FCC sites byother cations, or the replacement of every n layers of lanthanidecations with a substituent cation, in the FCC cation sublattice.

The topotactic anion exchange process can be used for the production ofperovskite films, which are desirable for many applications. Perovskitesconsist of two interpenetrating cubic cation sublattices, resulting in aBCC cation lattice. A perovskite precursor can be transformed under theproper thermal and oxidizing conditions to yield an epitaxial perovskitefilm. In particular, because oxygen mobility in some perovskites islimited, a metal-nonmetal perovskite (LnBX₃ or other BCC-cation lattice)precursor can be partially converted to a metal-oxide perovskiteproduct, such that a layered structure of LnBO₃/LnBX₃/substrate results.This can provide the benefit of having a conductive bottom electrode,LnBX₃, topotactic with an insulating top layer (LnBO₃). Furthermore,this effect can be exploited for systems where different cationicconstituencies are desired in the LnBX₃ and Ln′B′O₃ films, by depositinga Ln′B′O₃ film epitaxially on a LnBX₃ film. The likely result would be atopotactic LnBO₃ in between the epitaxial Ln′B′O₃ and the LnBX₃.

Selection of anions for a BCC-cation-sublattice system is more criticalthan for an FCC-cation-sublattice system, because the cation sublatticeis not close-packed, and is stabilized in large part by the anions.BCC-based layered structures, such as the Ruddlesden-Popper phases,Ln_(n+1)+B_(n)O_(3n+1), can be synthesized by the same approach. Thepreferred anions will have both average charge and average ionic radiusclosely approximating that of oxygen, as described above. The tolerancefactor, t, is an effective tool for predicting compositions that canform a specific phase. For perovskites of the formula ABX₃, thetolerance factor equation is R_(X)+R_(A)=1.414*t*(Rx+R_(B)), there R isthe ionic radius for 12-coordinated A, 6-coordinated B, or 6-coordinatedX, and cubic perovskites are observed for values of t from 0.90 to 1.0.For values of t less than 0.90, down to 0.72 or less, perovskites havingslight distortion from cubic are observed, either orthorhombic ormonoclinic. Table W lists the calculated tolerance factors for 3-3perovskites, where A is the Ln having the greatest ionic radius, La(R_(A)=1.36 Å) in 12-coordination, for anionic components oxygen, 50:50nitrogen-fluorine, and sulfur. Table V lists the calculated tolerancefactors for 3-3 perovskites, where A is the Ln having the least ionicradius, Lu (R_(A)=1.03 Å) in 12-coordination, for anionic componentsoxygen, 50:50 nitrogen-fluorine, and sulfur. Table VI lists thecalculated tolerance factors for 4-2 perovskites where A is Ce(R_(Ce)=1.14 Å) for anionic components oxygen, 50:50 nitrogen-fluorine,and sulfur.

TABLE IV Tolerance factors (t) for 3-3 LaBX₃ compositions, where R_(La)= 1.36, X = O, (N, F), or S, and R_(B) is the ionic radius of the cationin Å. B R_(B) t O t N, F t S Al 0.54 0.89 0.89 0.85 Co 0.58 0.87 0.870.84 Ni 0.58 0.87 0.87 0.84 Fe 0.60 0.86 0.86 0.83 Cr 0.62 0.85 0.850.83 Mn 0.62 0.85 0.85 0.83 Ga 0.62 0.85 0.85 0.83 V 0.64 0.84 0.84 0.82Ti 0.67 0.83 0.83 0.81 Rh 0.67 0.83 0.83 0.81 Ru 0.68 0.83 0.83 0.81 Ir0.68 0.83 0.83 0.81 Mo 0.69 0.82 0.82 0.80 Nb 0.72 0.81 0.81 0.79 Sc0.75 0.80 0.80 0.78 Pd 0.76 0.80 0.80 0.78 In 0.80 0.78 0.78 0.77 Lu0.86 0.76 0.76 0.75 Yb 0.87 0.76 0.76 0.75 Tm 0.88 0.75 0.75 0.75 Tl0.89 0.75 0.75 0.74 Er 0.89 0.75 0.75 0.74 Y 0.90 0.75 0.75 0.74 Ho 0.900.75 0.75 0.74 the existence of perovskite phases is predicted for thaving values in the range of at least 0.72 to 1.0. Note that LaAlO₃,with a t = 1.01, is known to exist as a distorted perovskite.

TABLE V Tolerance factors (t) for 3-3 LuBX₃ compositions, where R_(Lu) =1.03, X = O, (N, F), or S, and R_(B) is the ionic radius of the cationin Å. B R_(B) t O t N, F t S Al 0.54 1.01 1.01 0.95 Co 0.58 0.99 0.990.94 Ni 0.58 0.99 0.99 0.94 Fe 0.60 0.98 0.98 0.93 Cr 0.62 0.97 0.970.92 Mn 0.62 0.97 0.97 0.92 Ga 0.62 0.97 0.97 0.92 V 0.64 0.96 0.96 0.91Ti 0.67 0.94 0.94 0.90 Rh 0.67 0.94 0.94 0.90 Ru 0.68 0.94 0.94 0.90 Ir0.68 0.94 0.94 0.90 Mo 0.69 0.93 0.93 0.89 Nb 0.72 0.92 0.92 0.88 Sc0.75 0.91 0.91 0.87 Pd 0.76 0.90 0.90 0.87 In 0.80 0.89 0.89 0.86 Lu0.86 0.86 0.86 0.84 Yb 0.87 0.86 0.86 0.84 Tm 0.88 0.86 0.86 0.83 Tl0.89 0.85 0.85 0.83 Er 0.89 0.85 0.85 0.83 Y 0.90 0.85 0.85 0.83 Ho 0.900.85 0.85 0.83 the existence of perovskite phases is predicted for thaving values in the range of at least 0.72 to 1.0.

TABLE VI Tolerance factors (t) for 4-2 CeBX₃ compositions, where R_(Ce)= 1.14, X = O, (N, F), or S, and R_(B) is the ionic radius of the cationin Å. B R_(B) t O t N, F t S Fe 0.69 0.86 0.86 0.83 Ni 0.69 0.86 0.860.83 Co 0.70 0.86 0.86 0.83 Cu 0.73 0.84 0.84 0.82 Zn 0.74 0.84 0.840.82 Mn 0.75 0.84 0.84 0.81 Cr 0.77 0.83 0.83 0.81 V 0.79 0.82 0.82 0.80Pt 0.80 0.82 0.82 0.80 Pd 0.86 0.79 0.79 0.78 Cd 0.95 0.76 0.76 0.76 Hg1.00 0.75 0.75 0.74 Pb 1.20 0.69 0.69 0.69 the existence of perovskitephases is predicted for t having values in the range of at least 0.72 to1.0.

g. Substrate Components for Chemical Stability

It is important that the precursor is stable in contact with thesubstrate, either kinetically, but preferably chemically. The chemicalcompatibility of most candidate systems has not been determined, so thismust be done either experimentally or through calculation. It is likely,however, that because the cation sublattice is observed to be rigid, itwill likely have low reactivity.

The precursor composition can preferably have a cationic component thatis the same as the substrate. For example, silicon-containing phases fora film on silicon. Specifically, silicon-containing oxynitrides such asCaFe₂O₄-type, Scheelite, pseudowollastonite, and garnet have beenpredicted to exist. The addition of the substrate cationic component tothe precursor and product LnA/O_(x) film increases the likelihood ofinterfacial thermodynamic stability. Oxygen has been demonstrated to beat least partially replaced by nitrogen in olivine-type andbeta-K₂SO₄-type oxides.

2. Anion

Rocksalt-based and fluorite-based LnX_(y) and some rocksalt-basedTrX_(y) have very high intersolubility, so extensive anion alloying ispossible. For example, in a TrXy system, a single solid solutionHf(C,N,O) phase exists for HfC+HfN˜<=30% (combined). Above this, atwo-phase mixture of HfO and Hf(N,C) is observed, but these two phasesare topotactically compatible, such that they consist only regions withdiffering anion ordering. An infinite variety of anion stoichiometriesexists, such that it is not possible to enumerate all possibilities. Thefollowing gives guidance for the selection of appropriate anionstoichiometry for a given system.

The most important aspect of this is that the lattice parameter can betailored to minimize the change in lattice dimension upon conversion tofull oxide. Note that the relative changes in lattice dimension aremostly for pure compounds. For anion alloys, this can be reduced. Forexample, the nitrogen to fluorine ratio of nitrofluorides also can betailored to yield a nearly zero change in lattice dimension uponconversion. Nitride fluorides can be regarded as pseudo-oxides, becausethe charge of a fluoride and a nitride is equivalent to two oxides, andall have similar ionic radii. Many nitride-oxide-fluorides have beenfound, all having very similar fluorite-derived crystalline structures,and very similar cube-lattice dimensions. Carbon can be furthersubstituted for nitrogen. Anion doping can also control anion mobilitiesin the precursor. Nitrogen in ZrO₂ creates oxygen vacancies.

3. Optimization

It is clear from the preceding that it is possible to tailor the latticeparameter to an appreciable extent for both precursor and product(oxide) phase. It is possible for one skilled in the art and using thismethod to devise a good lattice fit to many different substrates andfilms for both precursor and product, eliminating strain-related defectsthat may be introduced during conversion if the change in latticedimension is otherwise large.

K. Conversion Process

The anionic diffusivities of compositions can typically be found in theliterature, as the result of computer simulation studies of oxygenmigration, or can be interpolated from knowns or predicted based onempirical rule sets. In any case, films resulting from a particularprocess will exhibit some variation due to microstructural differences,and these must be taken into account. For any given system, parametersmust be optimized.

The topotactic oxidation reaction for a given system must take placewithin a certain temperature range in order to maximize the crystallinequality of the product. If the temperature is too low, pores or cracksmay form, for example if the mobility of one anionic component issignificantly larger than that of the other. At excessively hightemperatures, the reaction may occur too quickly for a topotacticoxidation front to be maintained. Or, excessive amounts of oxygen maydiffuse through the oxidized layer to react with the oxidation-sensitivesubstrate. Additionally, there is a critical thickness below whichstress-induced mosaic is less likely to form.

Electrical resistivity is a good primary measure of the appropriatetemperature for transformation. In log σ vs 1/T plots, a break isobserved around 550-600° C. for lanthanide oxides. This is the regionwhere ionic conductivities begin to contribute appreciably to theelectrical conductivity. Anion exchange takes place preferably between 0and 1000° C., more preferably between 200 and 800° C., and mostpreferably between 300 and 600° C.

L. LnO_(z) Properties

1. Electrical

Lanthanide oxides are fairly electrically conductive. Thus, they canserve as a bottom electrode in many devices It may be necessary in othercases to treat the product films in an oxygen ambient, and theconsequences of high temperature vacuum annealing on these materials isa process consideration. At very low pressures, sesquioxides are n-typesemiconductors, with σ proportional to p_(O2) ^(−1/6), determined fromthe reaction: P₀ ²⁻<--->½O₂ (g)+[ ]₀+2 e⁻.

It is important to note the electrical conductivity of LnO₂ films inrelation to the conditions for processing of functional films. It isthis final oxygen content that will determine the resistivity of atemplate film, which may or may not be used as a conductive bottomelectrode. An example from the art of the wide range of resistivitiesobserved in a LnO_(z) film as a function of oxygen content is shown inthe art in FIG. 20 for CeO₂. Hysteresis of the oxygen content and thusresistivity of LnO_(z) films must also be taken into account in relationto the processing conditions for functional oxide films using saidLnO_(z) layers. Trapped charge is relaxed out at ˜350° C., and oxygenmobility begins to contribute appreciably to charge transport at ˜500°C.

Electrical conductivity in Ln_(n)O_(2n-2) single crystals varies greatlywith n, over eight orders of magnitude. LnO_(z) phases are mixed valencesemiconductors, p-type from z=1.5-1.75, and n-type from z=1.75-2.Conductivity is highest in LnO_(z) compositions nearest z=1.75, for bothordered and disordered phases, which would have a roughly equaldistribution of Ln³⁺ and Ln⁴⁺ ions and a maximum of electronic disorder.p-type conductivity arises from oxygen excess, with a proportional top_(O) ₂ ^(+1/5.4±0.1) for Ln=Ho, Eu, and Y. The electrical conductivityor LnO_(z) shows a knee at 550-600° C. in a vs 1/T plots, indicating asignificant ionic contribution to transport above those temperatures.FIG. 20 shows a change in room-temperature conductance of five orders ofmagnitude for a CeO₂ crystal reduced by heating in vacuum. Electricaltransport properties of an epitaxial LnO_(z) film are controlledstrongly by the processing conditions. It is expected that in mostcases, a separate oxygen anneal will not be required to stabilize theproperties of the LnO_(z) template film.

For visual indication, color may be a good indication of the oxidationstate of the product films. Stoichiometric lanthanide sesquioxides arelight pastel in color, except for PrO₂ and TbO₂, which are black andred. Mixed valence intermediate oxides, and the higher oxides of Ce, Prand Tb, are all deep colors.

Precursor conductivity may also be important. LnH₂ are metallicconductors (except EuH₂ and YbH₂). Better conductors than the metals byat least 2× each. Nitrides are known to be good electrical conductors.

2. Thermodynamic Stability of LnO_(z)

The lanthanide oxides are some of the most thermodynamically stableoxide compounds known. Their free energies of formation are on the orderof those of Al₂O₃ and CaO, as is known and shown in the art in FIG. 21.This implies that it may be possible in many cases to grow lanthanideoxides or reduced lanthanide oxides directly on elemental or otheroxidation-sensitive substrates, although this is always determined bythe specifics of a given film deposition process. It is also possible togrow an anion-alloyed oxide on a sensitive substrate, which can later betopotactically transformed to the full oxide.

3. Magnetism

Most LnX_(y) have a nonzero magnetic moment, which may be an importantfactor in selecting an appropriate composition for a given application.It is likely that the preferred elements for use in the creation ofexplicitly nonmagnetic layers will be lanthanum and lutetium, and theiralloys with scandium, yttrium, zirconium, and hafnium. This does notexclude, however, the other lanthanides, as a template film created bythis technique may be so thin as to obviate the consideration, oralternatively magnetism may not be a concern in some cases of the use ofsuch a template layer. It is considered that only an ultra-thin film maybe needed in order to obtain the oxide templating functionality. In suchcases, adverse impact of the magnetic susceptibility of the templatelayer are mitigated.

Magnetic moments don't change appreciably on hydridization, indicatingthat the f electrons don't take part in bonding with hydrogen. But,decreased magnetic ordering upon hydridization indicates removal ofconduction band electrons. For example, all LnH₂ and LnH₃ are magnetic,except for La and Lu, and Yb, with low magnetic susceptibilities.

4. Orientation Control

The orientation of films during deposition is controlled via the usualmeans for epitaxial deposition. Some additional concerns exist.Orientation control of nitrides through deposition conditions ispossible, regardless of substrate orientation, due to surfaceenergetics. LnO_(z) are known to grow (110) on (100) silicon, eventhough (100) would be a better lattice fit. The topotactic anionexchange method may be an easy route to (100) LnO_(z)/(100)Si.

5. Electronic Defect Relaxation

CeO₂ crystals retain a high electrical conductivity and highconcentration of color centers after re-oxidation. But, on heating to350° C. or higher in air is required to relax the trapped charge,reducing electrical conductivity and removing color centers. See FIG.20.

6. Atmospheric Stability

Reduced lanthanide oxides are very hygroscopic. For example, CeO_(1.67)exposed to air at room temperature reoxidized to nearly CeO₂ in a rapidexothermic reaction. The same reaction in a high-humidity atmosphereresults in disintegration of the crystal into powder perhaps due to theformation of a hydroxide, or more likely because the stable form isA-type. Thus, it is anticipated that the most reasonable implementationsof the method will involve in situ anion exchange.

7. Tell-Signs

It is possible to detect whether a device has been manufactured usingthe topotactic anion exchange method, versus a modified direct oxidedeposition technique. The major tell-sign is that there will be somefraction of the original anionic component remaining in the film. Thiswill vary with the exact cationic composition and lattice structure of afilm in question, but the value can be determined from the stoichiometryof the cationic components, as determined experimentally, consideringthe solubility limits for the particular system. In some cases, theanionic remainder may vary, based on the thermal and atmospheric historyof a film. Even in the case of thermal and atmospheric treatmentsdesigned to mask the process by which a film was grown and converted,trace amounts of the initial anionic species will remain, and these canbe detected by a chemical probe technique having high spatialresolution. Other microstructural tell-signs are also left behind afterthe conversion process, and a microstructural examination of such afilm, for example by transmission electron microscopy, specificallyusing a chemical analysis technique such as electron energy dispersivespectroscopy, will reveal these features.

The presence or absence of an FCC cation sublattice can be detected bystandard x-ray or electron diffraction techniques, specifically, by thepresence or absence of certain reflections. Intensity is dominated bythe cation sublattice. Lanthanides have a much higher scatteringcross-section than oxygen or other nonmetal components, and diffractedintensity is modulated more strongly by the lanthanide components thanby the lighter, nonmetal (anionic) components. In some cases, slightlattice distortions will also modulate diffracted intensity, and theeffects can be separated, as diffraction physics are well understood.Additionally, the applicable crystal structures are known, and otherscan be deduced or determined from diffraction data.

M. General Method of Fabrication

Those skilled in the art of thin film deposition techniques canduplicate the method by using the appropriate source materials, and bycontrolling the timing of non-metallic species bearing gas delivery. Thegeneral method of fabrication differs from the usual methods of oxidethin film deposition in the composition of source materials, and in thetiming of delivery of material containing the non-metallic components ofthe film, and in the subsequent thermal treatment under oxidativeconditions to exchange the non-metallic species for oxygen in the film.The usual methods of oxide film fabrication are chemical vapordeposition (CVD), pulsed laser deposition (PLD), chemical solutiondeposition (CSD), molecular beam epitaxy (MBE), sputtering, and so on.

Depending upon the specific film growth technique, an overpressure ofthe appropriate vapor source bearing non-metallic species will berequired to stabilize the non-oxide or low-oxide precursor material,which will vary according to the deposition technique and depositiontemperature. For embodiments of this invention, an infinite variety ofpossible precursor and product compositions exists, such that it is notpossible to enumerate all of the possibilities, but a given system canbe optimized by one skilled in the art. The lanthanide and other metalsare delivered in the fashion usual for each given technique, and inratios chosen to result in a film of the desired stoichiometry. A widerange of process parameters is applicable for this variety ofcompositions. The non-metallic components can be delivered together withthe metallic components from the same source or separate from them, orin some cases both. Generally, thermodynamic considerations and directexperimentation will be required to determine the optimal processparameters, such as substrate temperature for deposition, substratetemperature for topotactic anion exchange, cation stoichiometry,non-metallic stoichiometry, feed gas overpressures, if any, vacuum levelfor various stages, time required for anion exchange, and the processparameters specific to the techniques. The approximate processparameters for the deposition stage via each given deposition techniquewill be apparent to those skilled in the art, and ample examples ofdeposition parameters for oxide and non-oxide films are available in theliterature.

A film deposition system typically consists of a vacuum chamber, aheated substrate stage, and a vapor source. The substrate is mounted onthe heated stage. The air is evacuated from the system to a vacuum levelappropriate to the deposition method, which can range from ambientpressures for CSD to about 1E-10 Torr for some MBE. The substrate stageis heated to the desired temperature, and sometimes some type ofpre-deposition thermal and chemical treatment is performed on thesubstrate, usually to remove surface impurities. An appropriatebackground gas feed is begun prior to deposition, if desired, in orderto stabilize the desired product film phase. This can consist of any ofa number of gases, for example oxygen, nitrogen, fluorine, or carbonmonoxide. The lanthanide and other metals, and in some cases also someportion of the non-metallic components, are then delivered to the targetby an appropriate vaporization technique—for example by laser ablationof a solid target puck, by sputtering of a solid target puck, byevaporation and delivery of a chemical vapor source, or by evaporationfrom elemental sources. Once deposition has completed, the sources areterminated or closed, and the substrate stage is cooled, typically withan overpressure of gases that stabilizes the desired product phase. Insome methods, the substrate is quenched.

Topotactic anion exchange occurs subsequent to film deposition,preferably in the same chamber, but sample storage between depositionand anion exchange is possible. The substrate is heated to the desiredtemperature, preferably in an ambient that maintains the stability ofthe precursor, but heating may also be in some other ambient, such as anoxidizing one. Once the desired temperature is reached, feed gases ofappropriate ratios are introduced into the chamber, such that topotacticanion exchange occurs to result in the desired oxide film. In apreferred embodiment, the topotactic anion exchange is performedimmediately after deposition, once the substrate is cooled to thedesired exchange temperature. In yet another preferred embodiment, thebackground gases are changed smoothly over a period of time from thosefor which the precursor is stable to ones for which the product oxide isstable, such that the exchange takes place in a continuous, rather thanan abrupt, manner.

It is important to note that low oxides of lanthanides and mostlanthanide salts are susceptible to reaction with air and water, so thatfor analysis of precursors, such as by x-ray diffraction, must beperformed after either applying a capping layer, or under inert gas orvacuum. Such methods will be familiar to those skilled in the art. Forfeed gases bearing the non-metallic film components, overpressuresspanning a very wide array of pressures are possible, and will depend onthe chemistry of a given situation. In some cases, more active gasesthan elemental gases may be required.

N. Possible Embodiments of the Invention

The mechanism of topotactic anion exchange is broadly applicable in awide variety of applications, and enables a large number of new devices,consisting of layers of specific composition and morphology. Thisinvention entails typically a single deposition of a layer that ischemically stable in contact with the substrate at the time ofbarrier-layer deposition, and after a brief oxidation anneal, is stablein contact with the epitaxial oxide film deposited thereon at the timeof its deposition. Individual crystalline grains of precursor materialconverted to the product oxide retain their crystalline texture andorientation. Thus, precursors of many different crystalline textures canbe used for the fabrication of devices. A matrix of some possible devicegeometries is shown in FIG. 22. These are given by way of illustrationof the invention, and not by way of limitation thereof. The method offabrication and compositions of matter used here to fabricate suchdevices are in all cases believed to be covered by this disclosure, tothe extent that fabrication utilizes anion exchange topotaxy. Figureabbreviations are as follows: amorph—amorphous, biax—biaxially textured,epi—epitaxial, fiber—fiber textured, Ln—lanthanide or lanthanides,M—metal or metals, polyx—polycrystalline, single xtal—singlecrystalline, topo—topotactic, X—anion or anions, xtal—crystalline.

Note that for purposes of a substrate surface, an epitaxial film isequivalent to a single crystal. All of the devices are believed to beuseful as an oxide template and barrier layer for subsequent depositionof oxides, and in some cases have separate functionality themselves.Some items are left blank but this is not an indication of absence ofparticular embodiments, rather, because the number of possibleembodiments and their utility is effectively infinite, no attempt toenumerate every possibility is attempted, and specific functionality isnoted only for selected items. Many embodiments, or the functionalitythey provide, are applicable to many others, as will be obvious to oneskilled in the art. For example, (t.1) would also be useful with asingle crystal surface, or in some cases a polycrystalline surface. Manyembodiments are also useful on non-metal surfaces, for example as anoxygen barrier on a previously-deposited but oxygen-permeable oxidelayer.

A description of some possible embodiments is listed below. Note thatthe brief descriptions are given for only selected embodiments, and aremeant to indicate the general usefulness of the method, and to describesome specific device geometries.

(a) Partially converted epitaxial lanthanide oxide film on a singlecrystal surface

-   -   (a.1) Ln₂O₃/Ln(N,O,F)/Si        -   (a.1.ii) Eu₂O₃/Eu(N,O,F)/Si—epitaxial fit of precursor and            product for an oxide template layer on silicon. Precursor            can be used as an electrode        -   (a.1.iii) Eu2O3/EuO/Si—Eu2O3 as converted from EuO

(b) Fully converted epitaxial lanthanide oxide film on a single crystalsurface

-   -   (b.1) Gd(O,F)/Si for epitaxial fit for both precursor and        product    -   (b.2) Eu2O3/Si—as converted from EuO/Si    -   (b.3) Eu2O3/Si—as converted from Eu(N,O,F)/Si    -   (b.4) La2O3/Si—as a gate oxide, or epitaxial template layer for        deposition of functional oxides    -   (b.5) Ln2O3/Si—useful for fabrication of Ln2O3 MEMS devices, or        for epitaxial template layer for oriented-oxide MEMS devices    -   (b.6) Ln2O3/Si—for fabrication of oxide membranes    -   (b.7) Ln2O3/Si—from Ln(N,P) to minimize change in lattice        parameter upon conversion    -   (b.8) Ln2O3/Si—from Ln(N,F) to minimize change in lattice        parameter upon conversion    -   (b.9) Ln2O3/Si—from Ln(C,N,O,F,P,S), as a component in an        all-oxide electronic device stack    -   (b.10) Ln2O3 (001)/Si (001)—as a method to achieve (001) Ln2O3        on (001) Si without the need for employing a vicinal surface    -   (b.11) Ln2O3/Si—as an epitaxial oxide template for deposition of        a ferroelectric material for an epitaxial ferroelectric        capacitor structure.

(c) Directly deposited epitaxial reduced lanthanide oxide-salt film on asingle crystal surface

(d) Partially converted biaxially textured lanthanide oxide film on abiaxially textured surface

-   -   (d.1) CeO₂/CeN/Ni—good epitaxial fit for precursor and surface,        and for product oxide and YBCO for subsequent deposition thereon    -   (d.2) CeO₂/CeN/Cu—good epitaxial fit for precursor and surface,        and for product oxide and YBCO for subsequent deposition thereon

(e) Fully converted biaxially textured lanthanide oxide film on abiaxially textured surface

-   -   (e.1) CeO₂/Ni—from CeN, good epitaxial fit for precursor and        surface, and for product oxide and YBCO for subsequent        deposition thereon    -   (e.2) CeO₂/CeN/Cu—from CeN, good epitaxial fit for precursor and        surface, and for product oxide and YBCO for subsequent        deposition thereon    -   (e.3) Lu₂O₃/Lon/Ni or Lu₂O₃/Lon/Cu—good epitaxial fit of product        to substrate

(f) Directly deposited biaxially textured reduced lanthanide oxide-saltfilm on a biaxially textured surface

(g) Partially converted fiber-textured lanthanide oxide film on a fibertextured surface

(h) Fully converted fiber-textured lanthanide oxide film on a fibertextured surface

-   -   (h.1) Lu₂O₃/Pt—from Lon, fiber textured platinum present on the        surface of a three-dimensional object, such as a turbine blade,        provides an epitaxial template and adhesion layer to oriented        thermal barriers, such as ZrO₂ or layered perovskite-based        materials having low thermal conductivity, providing good        epitaxial fit to ZrO₂ and orientation control of layered oxide.    -   (h.2) Dy₂O₃/Pt—from DyC₂, fiber textured platinum present on the        surface of a three-dimensional object, such as a turbine blade,        provides an epitaxial template and adhesion layer to oriented        thermal barriers, such as ZrO₂ or layered perovskite-based        materials having low thermal conductivity, providing good        epitaxial fit to ZrO₂.

(i) Directly deposited fiber-textured reduced lanthanide oxide-salt filmon a fiber textured surface

(j) Partially converted oriented lanthanide oxide film on apolycrystalline or amorphous (non-templating) surface

-   -   (j.1) Oriented precursor films of Ln(C,N,F,P,S), etc. can be        deposited by techniques such as ion beam assisted deposition        (IBAD), or by controlling deposition parameters such that the        precursor adopts a certain orientation due to process        parameters, as is observed for some nitrides. The product gains        have the same orientation as the grains of the precursor from        which they are converted.

(k) Fully converted oriented lanthanide oxide film on a polycrystallineor amorphous (non-templating) surface

-   -   (k.1) Oriented precursor films of Ln(C,N,F,P,S), etc. can be        deposited by techniques such as IBAD, or by controlling        deposition parameters such that the precursor adopts a certain        orientation due to process parameters, as is observed for some        nitrides. The product gains have the same orientation as the        grains of the precursor from which they are converted.

(l) Directly deposited oriented reduced lanthanide oxide-salt film on apolycrystalline or amorphous (non-templating) surface

-   -   (1.1) Oriented precursor films of Ln(C,N,O,F,P,S), etc. can be        deposited by techniques such as ion beam assisted deposition        (IBAD), or by controlling deposition parameters such that the        precursor adopts a certain orientation due to process        parameters, as is observed for some nitrides. The product gains        have the same orientation as the grains of the precursor from        which they are converted.

(m) Partially converted polycrystalline lanthanide oxide film on apolycrystalline or amorphous (non-templating) surface

(n) Partially converted polycrystalline lanthanide oxide film on apolycrystalline or amorphous (non-templating) surface

-   -   (n.1) Ln₂O₃ from, e.g., LnSe, as a fuel cell electrode. A        composition having a large change in lattice parameter through        conversion, to intentionally introduce porosity, can increase        the surface area and enhance the efficiency of such a layer.

(o) Directly deposited polycrystalline reduced lanthanide oxide-saltfilm on a polycrystalline or amorphous (non-templating) surface

(p) Partially converted epitaxial lanthanide-metal oxide film on asingle crystal surface

-   -   (p. 1) ABO₃/ABX_(y)/Si—Perovskite precursor and product as an        epitaxial buffer layer and oxygen barrier, cation barrier, and        potentially bottom electrode. Oxygen vacancy diffusion in some        perovskites has been demonstrated to be very low. Only the        surface must be oxidized, perhaps as little as a single        monolayer, or several unit cells. Some nitride perovskites are        known.

(q) Fully converted epitaxial lanthanide-metal oxide film on a singlecrystal surface

-   -   (q.1) (La,Y)₂O₃/Si as a nonmagnetic epitaxial oxide template or        as a surface for silicon-on-insulator silicon deposition    -   (q.2) LnMnO₃/Si—as an epitaxial multiferroic    -   (q.3) LnMn₂O₅/Si—as an epitaxial multiferroic    -   (q.4) (Ln,W)₂O₃/Si—to enable integration of epitaxial phosphor        materials and laser hosts with silicon and other single crystal        substrates    -   (q.5) (Ln,Si)₂O₃/Si—to enhance chemical stability of film with        substrate

(r) Directly deposited epitaxial reduced lanthanide-metal oxide-saltfilm on a single crystal surface

-   -   (r.1) (La,Zr)(N,O/Si—useful as a bottom electrode and epitaxial        oxide template

(s) Partially converted biaxially textured lanthanide-metal oxide filmon a biaxially textured surface

-   -   (s.1) (La,Zr)₃O₇/(La,Zr)N/Ni—good epitaxial fit of product to        YBCO for subsequent deposition thereon.

(t) Fully converted biaxially textured lanthanide-metal oxide film on abiaxially textured surface

-   -   (t.1) (Ce, Ca)₂O₃/Ag—from (Ce, Ca)₂(C,N,O,F,P,S) to minimize        temperature required for deposition, for substrates with low        melting temperatures    -   (t.2) (Ce,Y)₂O₃/Ni—from (Ce,Y)₂(C,N,O,F,P,S) to utilize doping        to suppress phase transitions in product phase, stabilizing the        cubic form

(u) Directly deposited biaxially textured reduced lanthanide-metaloxide-salt film on a biaxially textured surface

(v) Partially converted fiber-textured lanthanide-metal oxide film on afiber textured surface

(w) Fully converted fiber-textured lanthanide-metal oxide film on afiber textured surface

(x) Directly deposited fiber-textured reduced lanthanide-metaloxide-salt film on a fiber textured surface

(y) Partially converted oriented lanthanide-metal oxide film on apolycrystalline or amorphous (non-templating) surface

(z) Fully converted oriented lanthanide-metal oxide film on apolycrystalline or amorphous (non-templating) surface

(aa) Directly deposited oriented reduced lanthanide-metal oxide-saltfilm on a polycrystalline or amorphous (non-templating) surface

(bb) or (ab) Partially converted polycrystalline lanthanide-metal oxidefilm on a polycrystalline or amorphous (non-templating) surface

(cc) or (ac) Partially converted polycrystalline lanthanide-metal oxidefilm on a polycrystalline or amorphous (non-templating) surface

(dd) or (ad) Directly deposited polycrystalline reduced lanthanide-metaloxide-salt film on a polycrystalline or amorphous (non-templating)surface

Other embodiments not related to specific substrate orientations arelisted below:

-   -   (1) Backscatter reduction in electron beam lithography    -   (2) LnO_(z) as an oxide surface for the deposition of epitaxial        films on silicon, especially for the purpose of allowing smaller        features to be patterned through the reduction in backscattering        of electrons used for electron beam lithography. This is        achieved through the use of an ultra-thin oxide template film.        The electron backscattering cross-section of silicon is much        lower than that of many oxides that are used as single        crystalline substrates for the deposition of epitaxial oxide        films. This benefit can also be realized in other cases, not        described in detail here, for which a biaxially textured,        polycrystalline, or even amorphous film is desired. The        proximity effect of electron beam lithography using this        technique is also commensurately reduced. LnO_(z) as an oxide        surface can be used in the reverse manner for metallic or other        substrates which have a high electron backscattering        cross-section. A thick LnO_(z) film of such a thickness that the        intensity of backscattered electrons is reduced over that which        would occur for bare metal, can be used to remove limitations on        resolution of backscattered electrons for films on such        substrates, including epitaxial, biaxially textured,        polycrystalline, and amorphous. The proximity effect of electron        beam lithography using this technique is also commensurately        reduced.    -   (3) A textured substrate for the deposition of an oxide        template, on which a layered complex oxide will grow with its        long axis perpendicular to the surface at all points on the        surface of a complex three-dimensional object. The most        significant application for such an oxide template film would be        for use as a thermal protection barrier for turbine blades. The        structure would consist of a complex three-dimensional object        such as a turbine blade as the substrate. On this substrate        surface, some combination of layers that would yield a fiber        textured layer on the surface would be deposited, for example a        bond-coat of Cr or MoCrAlY, with platinum deposited over that.        Annealing Pt under specific thermal conditions reproducibly        yields a Pt layer having (111) surface texture. Previous work        has demonstrated that a pseudocubic oxide, SrRuO₃, can be grown        on (111) Pt to adopt the texture of the Pt through local        epitaxy, and that a layered complex oxide will grow epitaxially        on it. Other methods of obtaining surface texture are possible.        This can be exploited in this example through the growth of a        LnX precursor, converting it to LnO_(z), and depositing an        appropriate layered complex oxide having the desired thermal and        other properties. Namely, in the case of thermal protection for        turbine blades, it should be stable at elevated temperatures and        have an appropriately low thermal conductivity. Layer order:        Layered complex oxide/fiber textured LnO_(z)/fiber textured        Pt/bond coat/3D object. The layered complex oxide need not        necessarily be oriented with its layering axis perpendicular to        the substrate surface, although this would provide the best        thermal protection. Even a 45°-oriented film would provide        better thermal protection than a polycrystalline protective        film, both due to the preferred orientation of the layers, and        also due to the elimination of randomly oriented grain        boundaries, which are paths for diffusion and transport. The        basis for choice of layered complex oxides for thermal        protection is based upon phonon scattering. Each layer in the        layered complex oxide acts as a scatterer for phonons (lattice        vibrations), which are a prime carrier of heat. This theory of        thermal resistance was developed by Kapitza. A second carrier of        heat is electrons, and this should also be taken into        consideration in selecting an appropriate layered complex oxide        for thermal protection. Additionally, it may in some cases be        advantageous to fabricate a structure that will conduct heat        very well in the direction perpendicular to a substrate surface,        but not parallel to it, for example in the case of heat sources        near thermally sensitive components. Another possible use of        this templating technique would be to design a structure that        yielded a layered complex oxide film having its layering axis in        the plane of the surface at all points of the surface of a        complex three-dimensional object. An additional benefit of this        technique is that having a fine grain size of platinum in the        fiber-textured platinum film, and thus of the template oxide        film on it, is that this would serve to limit the grain size of        the layered complex oxide film grown thereon. This can be an        important functionality because layered complex oxides tend to        have highly anisotropic growth rates, which can lead to        roughness in a film with the layering axis tilted away from the        surface normal.    -   (4) Ln2O3 can be present as the dielectric layer in a capacitor,        can function as an epitaxial oxide template layer for deposition        of a ferroelectric layer in an epitaxial stack structure such as        a capacitor.    -   (5) When a template surface is a cubic perovskite or other        structure presenting the same type of oxygen coordination on the        surface, epitaxy is facilitated.    -   (6) It is well known that layered crystalline phases and        superlattices possess much lower thermal conductivity than        isotropic and homogenous phases. It is also known that grain        boundaries decrease thermal migration as well. The mechanism for        all of these phenomena is called the Kapitza resistance, and is        the result of scattering of phonons by internal interfaces.        Superlattices and layered phases have extremely high densities        of interfaces, and thus lowered thermal conductivity. It is also        well known that a polycrystalline thin film of platinum (and        other noble metals) will adopt a preferred surface plane        orientation, and thus biaxial texture, after appropriate thermal        treatment. It is also known that it is possible to use this        biaxially textured platinum surface for the deposition of a        biaxially textured film, which adopts its texture via local        epitaxy with individual grains. Thus, it is possible to deposit        a LnX film on a complex three-dimensional object that has a        specific crystalline surface orientation at all points on the        three-dimensional surface. This can be used to deposit a layered        phase or superlattice material, on the object surface, that has        the same orientation with respect to the local surface tangent        at all points. The primary use for this device is in the        application of a thermal barrier coating on a device such as a        turbine blade. Layered phase are used for their low thermal        conductivity. This device would allow for the preferred        orientation of a layered phase, such as a Ruddlesden-Popper        phase, with a resultant dramatic increase in thermal insulative        performance. It may also be possible to deposit a lanthanide        oxide, transition metal oxide, or other metal oxide film        directly on a textured platinum surface. The oxide film would        preferably have a component of platinum or other noble metal,        for example La2Pt2O7. It may also be possible to deposit the        superlattice phase in the desired oriented fashion directly on a        textured platinum layer, although an adhesion layer is likely to        improve performance.    -   (7) Epitaxial, rocksalt-structured TrN (Tr=transition metal)        films have been investigated as barrier metals in devices on        silicon, particularly for ferroelectric capacitors, due to their        low electrical resistivity and low Schottky barrier. In typical        investigations of these barrier layers, efforts were made to        avoid oxidation of the nitride surface. Such layers could be        present beneath the devices described in the claims, to act as        barriers.    -   (8) Tungsten-doped LnO_(z) have uses as phosphors and as laser        hosts. Many lanthanide tungstates are known to have        fluorite-type structures, many with slightly distorted symmetry,        which could possibly be stabilized in a cubic form by epitaxy.        Even transformation to a non-cubic form could be acceptable, as        it could result in for example an oriented tetragonal structure        with c perpendicular to the substrate surface, or with a domain        structure in-plane, such as mixed tetragonal a and c, or slight        monoclinic distortions. With either of these domain structures        would probably be effectively equivalent to a cubic film, as the        distortions are small and they are not displacive, so that        domain walls should not have transport properties appreciably        different than the domains.

While the specific embodiments have been illustrated and described,numerous modifications can be made to the present invention, asdescribed, by those of ordinary skill in the art without significantlydeparting from the spirit of the invention. The breadth of protectionafforded this invention should be considered to be limited only by thescope of the accompanying claims.

1. A functional laminate structure, comprising: a substrate having asurface; and a crystalline layer comprising an oxide selected from thegroup of A(O,X)_(y), doped substituents thereof, anion-deficientversions thereof, and anion-excess versions thereof, wherein A is ametal, O is oxygen, X is a nonmetal constituent, y represents thestochiometry of O and X relative to A, and the crystalline layer is onthe surface.
 2. The structure of claim 1 wherein the surface issubstantially single crystalline.
 3. The structure of claim 2 wherein Acomprises a lanthanide element selected from the group consisting of Sc,Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu andcombinations thereof.
 4. The structure of claim 3 wherein thecrystalline layer is epitaxial with the surface.
 5. The structure ofclaim 4 wherein the surface is silicon.
 6. The structure of claim 4wherein the surface is not oxidized.
 7. The structure of claim 3 whereinA comprises Gd and X comprises F.
 8. The structure of claim 1 wherein Xcomprises fluorine.
 9. The structure of claim 1 wherein X comprisesnitrogen such that (O,X) comprises an oxynitride.
 10. The structure ofclaim 1 wherein X comprises fluorine and nitrogen.
 11. The structure ofclaim 1 wherein X comprises sulfur
 12. The structure of claim 1 whereinX comprises a nonmetal selected from the group consisting of N, F, S andcombinations thereof.
 13. The structure of claim 1 wherein X comprises anonmetal selected from the group consisting of H, C, N, F, P, S, Cl, Se,Br, Te and combinations thereof.
 14. The structure of claim 1 whereinthe crystalline layer has the structure of fluorite, bixbyite,pyrochlore, perovskite, layered perovskite, or scheelite type crystals15. The structure of claim 1 further comprising a metal-oxide layerpresent on the crystalline layer.
 16. The structure of claim 1 whereinthe oxygen content in the crystalline layer is less than 80%.
 17. Thestructure of claim 1 wherein the oxygen content in the crystalline layeris less than 50%.
 18. The structure of claim 1 wherein the oxygencontent in the crystalline layer is less than 20%.
 19. The structure ofclaim 1 wherein y is selected from the group of 0.33, 1, 1.5 and
 2. 20.The structure of claim 1 wherein the substrate is biaxially textured.21. The structure of claim 20 wherein A comprises a lanthanide elementselected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof.
 22. The structureof claim 20 wherein the substrate is nickel.
 23. The structure of claim1 further comprising a functional layer deposited on the crystallinelayer.
 24. The structure of claim 1 wherein the constituents of X havean average anionic charge that approximates that of oxygen.
 25. A methodof fabricating an epitaxial layer on an oxidation-sensitive substrate:providing an oxidizable substrate having a surface; heating thesubstrate; and depositing a crystalline film on the surface via vapordeposition comprising delivering to the substrate a gas bearing a metal(A), oxygen (O) and a nonmetal (X), the oxygen content of the gas beingsuch that the substrate does not oxidize at the temperature ofdeposition; wherein the deposited crystalline film comprises an oxideselected from the group of A(O,X)_(y), doped substituents thereof,anion-deficient versions thereof, and anion-excess versions thereof,wherein y represents the stochiometry of O and X relative to A.
 26. Themethod of claim 25 wherein the delivering to the substrate comprisesdelivering to the substrate the gas bearing the metal, the metalselected from the group of lanthanide elements consisting of Sc, Y, La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and combinationsthereof.
 27. The method of claim 26 wherein the providing an oxidizablesubstrate comprises providing the substrate such that the correspondinglanthanide oxide cannot be directly deposited on the surface withoutoxidizing the substrate.
 28. The method of claim 25 wherein thedelivering to the substrate comprises delivering to the substrate thegas bearing the nonmetal X, the nonmetal X selected from the groupconsisting of H, C, N, F, P, S, Cl, Se, Br, Te and combinations thereof.29. The method of claim 25 wherein the delivering to the substratecomprises delivering to the substrate the gas, the gas bearing thenonmetal X, the nonmetal X selected from the group consisting of N, F,S, and combinations thereof.
 30. The method of claim 25 whereindelivering to the substrate comprises delivering to the substrate thegas, the gas having a partial pressure of oxygen such that the substratedoes not oxidize.
 31. The method of claim 25 wherein delivering to thesubstrate comprises delivering to the substrate the gas, the gas bearinghydrogen and water such that the ratio of hydrogen to water in the gasis such that the substrate does not oxidize.
 32. The method of claim 25wherein the depositing a crystalline film comprises depositing thecrystalline film epitaxial to the surface.
 33. The method of claim 25further comprising exchanging the majority of the nonmetal X for oxygen.34. The method of claim 33 wherein the exchanging is topotactic.
 35. Themethod of claim 25 further comprising depositing a functional layer onthe crystalline film.
 36. The method of claim 25 wherein the providingan oxidizable substrate comprises providing the oxidizable substrate,the oxidizable substrate being biaxially-textured.